diff hunks that only consist of changes to context lines in the input diffs.

Thanks to the previous commit, lines like "## file => renamed-file ##"
are not considered context lines because they no longer have a leading space.

This gives some extra @@ lines because we now always calculate
two diffs: one for the patch metadata, like the commit message,
and another one for the actual file changes.
This is because the former contains lines with leading spaces that are not
context lines, so we never want to ignore them.
---
 range-diff.c          | 58 +++++++++++++++++++++++++-----
 t/t3206-range-diff.sh | 83 ++++++-------------------------------------
 2 files changed, 60 insertions(+), 81 deletions(-)

diff --git a/range-diff.c b/range-diff.c
index 72660453bd..df2147ef79 100644
--- a/range-diff.c
+++ b/range-diff.c
@@ -363,6 +363,31 @@ static void get_correspondences(struct string_list *a, struct string_list *b,
 	free(b2a);
 }
 
+static int are_diffs_equivalent(const char *a_diff, const char *b_diff) {
+	for (
+		const char
+			*a_eol = strchr(a_diff, '\n'),
+			*b_eol = strchr(b_diff,	'\n');
+		(a_eol = strchr(a_diff,	'\n')) &&
+		(b_eol = strchr(b_diff,	'\n'));
+		a_diff = a_eol + 1, b_diff = b_eol + 1
+	) {
+		if (!!a_eol != !!b_eol)
+			return 0;
+
+		// Ignore context lines.
+		if (a_diff[0] == ' ' &&	b_diff[0] == ' ')
+			continue;
+
+		size_t a_len = a_eol - a_diff;
+		size_t b_len = b_eol - b_diff;
+		if (a_len != b_len || strncmp(a_diff, b_diff, a_len))
+			return 0;
+	}
+
+	return 1;
+}
+
 static void output_pair_header(struct diff_options *diffopt,
 			       int patch_no_width,
 			       struct strbuf *buf,
@@ -390,8 +415,10 @@ static void output_pair_header(struct diff_options *diffopt,
 	} else if (!a_util) {
 		color = color_new;
 		status = '>';
-	} else if (strcmp(a_util->patch, b_util->patch)) {
-		color = color_commit;
+	} else if (a_util->diff_offset != b_util->diff_offset
+		   || strncmp(a_util->patch, b_util->patch, a_util->diff_offset)
+		   || !are_diffs_equivalent(a_util->diff, b_util->diff)) {
+		color =	color_commit;
 		status = '!';
 	} else {
 		color = color_commit;
@@ -436,13 +463,13 @@ static struct userdiff_driver section_headers = {
 		      "^.?@@ (.*)$", REG_EXTENDED }
 };
 
-static struct diff_filespec *get_filespec(const char *name, const char *p)
+static struct diff_filespec *get_filespec(const char *name, const char *p, size_t size)
 {
 	struct diff_filespec *spec = alloc_filespec(name);
 
 	fill_filespec(spec, &null_oid, 0, 0100644);
 	spec->data = (char *)p;
-	spec->size = strlen(p);
+	spec->size = size;
 	spec->should_munmap = 0;
 	spec->is_stdin = 1;
 	spec->driver = &section_headers;
@@ -450,11 +477,11 @@ static struct diff_filespec *get_filespec(const char *name, const char *p)
 	return spec;
 }
 
-static void patch_diff(const char *a, const char *b,
+static void patch_diff(const char *a, size_t size_a, const char *b, size_t size_b,
 		       struct diff_options *diffopt)
 {
 	diff_queue(&diff_queued_diff,
-		   get_filespec("a", a), get_filespec("b", b));
+		   get_filespec("a", a, size_a), get_filespec("b", b, size_b));
 
 	diffcore_std(diffopt);
 	diff_flush(diffopt);
@@ -467,6 +494,14 @@ static void output(struct string_list *a, struct string_list *b,
 	int patch_no_width = decimal_width(1 + (a->nr > b->nr ? a->nr : b->nr));
 	int i = 0, j = 0;
 
+	regex_t regex;
+	if (regcomp(&regex, "^ ", REG_EXTENDED | REG_NEWLINE))
+		BUG("invalid regex");
+	ALLOC_GROW(diffopt->ignore_regex, diffopt->ignore_regex_nr + 1,
+		   diffopt->ignore_regex_alloc);
+	diffopt->ignore_regex[diffopt->ignore_regex_nr] = &regex;
+	size_t ignoring_context_only_changes = diffopt->ignore_regex_nr + 1;
+
 	/*
 	 * We assume the user is really more interested in the second argument
 	 * ("newer" version). To that end, we print the output in the order of
@@ -504,9 +539,14 @@ static void output(struct string_list *a, struct string_list *b,
 			a_util = a->items[b_util->matching].util;
 			output_pair_header(diffopt, patch_no_width,
 					   &buf, &dashes, a_util, b_util);
-			if (!(diffopt->output_format & DIFF_FORMAT_NO_OUTPUT))
-				patch_diff(a->items[b_util->matching].string,
-					   b->items[j].string, diffopt);
+			if (!(diffopt->output_format & DIFF_FORMAT_NO_OUTPUT)) {
+				patch_diff(a_util->patch, a_util->diff_offset, 
+						b_util->patch, b_util->diff_offset, diffopt);
+			        diffopt->ignore_regex_nr = ignoring_context_only_changes;
+				patch_diff(a_util->diff, strlen(a_util->diff), 
+						b_util->diff, strlen(b_util->diff), diffopt);
+			        diffopt->ignore_regex_nr = ignoring_context_only_changes - 1;
+			}
 			a_util->shown = 1;
 			j++;
 		}
diff --git a/t/t3206-range-diff.sh b/t/t3206-range-diff.sh
index f875843b5e..9a63388bee 100755
--- a/t/t3206-range-diff.sh
+++ b/t/t3206-range-diff.sh
@@ -223,16 +223,7 @@ test_expect_success 'changed commit' '
 	      12
 	      13
 	      14
-	4:  $(test_oid t4) ! 4:  $(test_oid c4) s/12/B/
-	    @@ file
-	     @@ file: A
-	      9
-	      10
-	    - B
-	    + BB
-	     -12
-	     +B
-	      13
+	4:  $(test_oid t4) = 4:  $(test_oid c4) s/12/B/
 	EOF
 	test_cmp expect actual
 '
@@ -243,7 +234,7 @@ test_expect_success 'changed commit with --no-patch diff option' '
 	1:  $(test_oid t1) = 1:  $(test_oid c1) s/5/A/
 	2:  $(test_oid t2) = 2:  $(test_oid c2) s/4/A/
 	3:  $(test_oid t3) ! 3:  $(test_oid c3) s/11/B/
-	4:  $(test_oid t4) ! 4:  $(test_oid c4) s/12/B/
+	4:  $(test_oid t4) = 4:  $(test_oid c4) s/12/B/
 	EOF
 	test_cmp expect actual
 '
@@ -256,9 +247,7 @@ test_expect_success 'changed commit with --stat diff option' '
 	3:  $(test_oid t3) ! 3:  $(test_oid c3) s/11/B/
 	     a => b | 2 +-
 	     1 file changed, 1 insertion(+), 1 deletion(-)
-	4:  $(test_oid t4) ! 4:  $(test_oid c4) s/12/B/
-	     a => b | 2 +-
-	     1 file changed, 1 insertion(+), 1 deletion(-)
+	4:  $(test_oid t4) = 4:  $(test_oid c4) s/12/B/
 	EOF
 	test_cmp expect actual
 '
@@ -278,16 +267,7 @@ test_expect_success 'changed commit with sm config' '
 	      12
 	      13
 	      14
-	4:  $(test_oid t4) ! 4:  $(test_oid c4) s/12/B/
-	    @@ file
-	     @@ file: A
-	      9
-	      10
-	    - B
-	    + BB
-	     -12
-	     +B
-	      13
+	4:  $(test_oid t4) = 4:  $(test_oid c4) s/12/B/
 	EOF
 	test_cmp expect actual
 '
@@ -304,16 +284,14 @@ test_expect_success 'renamed file' '
 	    -    s/4/A/
 	    +    s/4/A/ + rename file
 	    Z
+	    @@
 	    -## file ##
 	    +## file => renamed-file ##
 	    Z@@
 	    Z 1
 	    Z 2
 	3:  $(test_oid t3) ! 3:  $(test_oid n3) s/11/B/
-	    @@ Metadata
-	    Z ## Commit message ##
-	    Z    s/11/B/
-	    Z
+	    @@
 	    -## file ##
 	    -@@ file: A
 	    +## renamed-file ##
@@ -322,10 +300,7 @@ test_expect_success 'renamed file' '
 	    Z 9
 	    Z 10
 	4:  $(test_oid t4) ! 4:  $(test_oid n4) s/12/B/
-	    @@ Metadata
-	    Z ## Commit message ##
-	    Z    s/12/B/
-	    Z
+	    @@
 	    -## file ##
 	    -@@ file: A
 	    +## renamed-file ##
@@ -348,8 +323,6 @@ test_expect_success 'file with mode only change' '
 	    -    s/4/A/
 	    +    s/4/A/ + add other-file
 	    Z
-	    Z## file ##
-	    Z@@
 	    @@ file
 	    Z A
 	    Z 6
@@ -364,8 +337,6 @@ test_expect_success 'file with mode only change' '
 	    -    s/11/B/
 	    +    s/11/B/ + mode change other-file
 	    Z
-	    Z## file ##
-	    Z@@ file: A
 	    @@ file: A
 	    Z 12
 	    Z 13
@@ -389,8 +360,6 @@ test_expect_success 'file added and later removed' '
 	    -    s/4/A/
 	    +    s/4/A/ + new-file
 	    Z
-	    Z## file ##
-	    Z@@
 	    @@ file
 	    Z A
 	    Z 6
@@ -405,8 +374,6 @@ test_expect_success 'file added and later removed' '
 	    -    s/11/B/
 	    +    s/11/B/ + remove file
 	    Z
-	    Z## file ##
-	    Z@@ file: A
 	    @@ file: A
 	    Z 12
 	    Z 13
@@ -434,9 +401,6 @@ test_expect_success 'changed message' '
 	    Z
 	    +    Also a silly comment here!
 	    +
-	    Z## file ##
-	    Z@@
-	    Z 1
 	3:  $(test_oid t3) = 3:  $(test_oid m3) s/11/B/
 	4:  $(test_oid t4) = 4:  $(test_oid m4) s/12/B/
 	EOF
@@ -453,9 +417,6 @@ test_expect_success 'dual-coloring' '
 	:     <RESET>
 	:    <REVERSE><GREEN>+<RESET><BOLD>    Also a silly comment here!<RESET>
 	:    <REVERSE><GREEN>+<RESET>
-	:     ## file ##<RESET>
-	:    <CYAN> @@<RESET>
-	:      1<RESET>
 	:<RED>3:  $(test_oid c3) <RESET><YELLOW>!<RESET><GREEN> 3:  $(test_oid m3)<RESET><YELLOW> s/11/B/<RESET>
 	:    <REVERSE><CYAN>@@<RESET> <RESET>file: A<RESET>
 	:      9<RESET>
@@ -466,16 +427,7 @@ test_expect_success 'dual-coloring' '
 	:      12<RESET>
 	:      13<RESET>
 	:      14<RESET>
-	:<RED>4:  $(test_oid c4) <RESET><YELLOW>!<RESET><GREEN> 4:  $(test_oid m4)<RESET><YELLOW> s/12/B/<RESET>
-	:    <REVERSE><CYAN>@@<RESET> <RESET>file<RESET>
-	:    <CYAN> @@ file: A<RESET>
-	:      9<RESET>
-	:      10<RESET>
-	:    <REVERSE><RED>-<RESET><FAINT> BB<RESET>
-	:    <REVERSE><GREEN>+<RESET><BOLD> B<RESET>
-	:    <RED> -12<RESET>
-	:    <GREEN> +B<RESET>
-	:      13<RESET>
+	:<YELLOW>4:  d966c5c = 4:  8add5f1 s/12/B/<RESET>
 	EOF
 	git range-diff changed...changed-message --color --dual-color >actual.raw &&
 	test_decode_color >actual <actual.raw &&
@@ -537,8 +489,6 @@ test_expect_success 'range-diff compares notes by default' '
 	    -    topic note
 	    +    unmodified note
 	    Z
-	    Z## file ##
-	    Z@@ file: A
 	EOF
 	test_cmp expect actual
 '
@@ -584,8 +534,6 @@ test_expect_success 'range-diff with multiple --notes' '
 	    -    topic note2
 	    +    unmodified note2
 	    Z
-	    Z## file ##
-	    Z@@ file: A
 	EOF
 	test_cmp expect actual
 '
@@ -644,11 +592,8 @@ test_expect_success 'format-patch --range-diff with --notes' '
 	    Z ## Notes ##
 	    -    topic note
 	    +    unmodified note
-	    Z
-	    Z## file ##
-	    Z@@ file: A
 	EOF
-	sed "/@@ Commit message/,/@@ file: A/!d" 0000-* >actual &&
+	sed "/@@ Commit message/,/unmodified note\$/!d" 0000-* >actual &&
 	test_cmp expect actual
 '
 
@@ -673,11 +618,8 @@ test_expect_success 'format-patch --range-diff with format.notes config' '
 	    Z ## Notes ##
 	    -    topic note
 	    +    unmodified note
-	    Z
-	    Z## file ##
-	    Z@@ file: A
 	EOF
-	sed "/@@ Commit message/,/@@ file: A/!d" 0000-* >actual &&
+	sed "/@@ Commit message/,/unmodified note\$/!d" 0000-* >actual &&
 	test_cmp expect actual
 '
 
@@ -709,11 +651,8 @@ test_expect_success 'format-patch --range-diff with multiple notes' '
 	    Z ## Notes (note2) ##
 	    -    topic note2
 	    +    unmodified note2
-	    Z
-	    Z## file ##
-	    Z@@ file: A
 	EOF
-	sed "/@@ Commit message/,/@@ file: A/!d" 0000-* >actual &&
+	sed "/@@ Commit message/,/unmodified note2\$/!d" 0000-* >actual &&
 	test_cmp expect actual
 '
 
