diff: teach --stat to ignore uninteresting modifications

When options such as --ignore-space-change are in use, files with
modifications can have no interesting textual changes worth showing.  In
such cases, "git diff --stat" shows 0 lines of additions and deletions.
Teach "git diff --stat" not to show such a path in its output, which
would be more natural.

However, we don't want to prevent the display  of all files that have 0
effective diffs since they could be the result of a rename, permission
change, or other similar operation that may still be of interest so we
special case additions and deletions as they are always interesting.

Signed-off-by: Matthew Rogers <mattr94@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -3153,16 +3153,19 @@ static void show_dirstat_by_line(struct diffstat_t *data, struct diff_options *o
 	gather_dirstat(options, &dir, changed, "", 0);
 }
 
+static void free_diffstat_file(struct diffstat_file *f)
+{
+	free(f->print_name);
+	free(f->name);
+	free(f->from_name);
+	free(f);
+}
+
 void free_diffstat_info(struct diffstat_t *diffstat)
 {
 	int i;
-	for (i = 0; i < diffstat->nr; i++) {
-		struct diffstat_file *f = diffstat->files[i];
-		free(f->print_name);
-		free(f->name);
-		free(f->from_name);
-		free(f);
-	}
+	for (i = 0; i < diffstat->nr; i++)
+		free_diffstat_file(diffstat->files[i]);
 	free(diffstat->files);
 }
 
@@ -3718,6 +3721,27 @@ static void builtin_diffstat(const char *name_a, const char *name_b,
 		if (xdi_diff_outf(&mf1, &mf2, discard_hunk_line,
 				  diffstat_consume, diffstat, &xpp, &xecfg))
 			die("unable to generate diffstat for %s", one->path);
+
+		if (DIFF_FILE_VALID(one) && DIFF_FILE_VALID(two)) {
+			struct diffstat_file *file =
+				diffstat->files[diffstat->nr - 1];
+			/*
+			 * Omit diffstats of modified files where nothing changed.
+			 * Even if !same_contents, this might be the case due to
+			 * ignoring whitespace changes, etc.
+			 *
+			 * But note that we special-case additions, deletions,
+			 * renames, and mode changes as adding an empty file,
+			 * for example is still of interest.
+			 */
+			if ((p->status == DIFF_STATUS_MODIFIED)
+				&& !file->added
+				&& !file->deleted
+				&& one->mode == two->mode) {
+				free_diffstat_file(file);
+				diffstat->nr--;
+			}
+		}
 	}
 
 	diff_free_filespec_data(one);

t/t4015-diff-whitespace.sh

@@ -789,7 +789,7 @@ test_expect_success 'checkdiff allows new blank lines' '
 	git diff --check
 '
 
-test_expect_success 'whitespace-only changes not reported' '
+test_expect_success 'whitespace-only changes not reported (diff)' '
 	git reset --hard &&
 	echo >x "hello world" &&
 	git add x &&
@@ -799,8 +799,42 @@ test_expect_success 'whitespace-only changes not reported' '
 	test_must_be_empty actual
 '
 
-test_expect_success 'whitespace-only changes reported across renames' '
+test_expect_success 'whitespace-only changes not reported (diffstat)' '
+	# reuse state from previous test
+	git diff --stat -b >actual &&
+	test_must_be_empty actual
+'
+
+test_expect_success 'whitespace changes with modification reported (diffstat)' '
 	git reset --hard &&
+	echo >x "hello  world" &&
+	git update-index --chmod=+x x &&
+	git diff --stat --cached -b >actual &&
+	cat <<-EOF >expect &&
+	 x | 0
+	 1 file changed, 0 insertions(+), 0 deletions(-)
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'whitespace-only changes reported across renames (diffstat)' '
+	git reset --hard &&
+	for i in 1 2 3 4 5 6 7 8 9; do echo "$i$i$i$i$i$i"; done >x &&
+	git add x &&
+	git commit -m "base" &&
+	sed -e "5s/^/ /" x >z &&
+	git rm x &&
+	git add z &&
+	git diff -w -M --cached --stat >actual &&
+	cat <<-EOF >expect &&
+	 x => z | 0
+	 1 file changed, 0 insertions(+), 0 deletions(-)
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'whitespace-only changes reported across renames' '
+	git reset --hard HEAD~1 &&
 	for i in 1 2 3 4 5 6 7 8 9; do echo "$i$i$i$i$i$i"; done >x &&
 	git add x &&
 	hash_x=$(git hash-object x) &&