diff: report modified binary files as changes in builtin_diff()

The diff machinery has two ways to detect changes to set the exit code:
Just comparing hashes and comparing blob contents.  The latter is needed
if certain changes have to be ignored, e.g. with --ignore-space-change
or --ignore-matching-lines.  It's enabled by the diff_options flag
diff_from_contents.

The code for handling binary files added by 1aaf69e669 (diff: shortcut
for diff'ing two binary SHA-1 objects, 2014-08-16) always uses a quick
hash-only comparison, even if the slow way is taken.  We need it to
report a hash difference as a change for the purpose of setting the
exit code, though, but it never did.  Fix that.

d7b97b7185 (diff: let external diffs report that changes are
uninteresting, 2024-06-09) set diff_from_contents if external diff
programs are allowed.  This is the default e.g. for git diff, and so
that change exposed the inconsistency much more widely.

Reported-by: Kohei Shibata <shiba200712@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -3673,6 +3673,7 @@ static void builtin_diff(const char *name_a,
 			emit_diff_symbol(o, DIFF_SYMBOL_BINARY_FILES,
 					 sb.buf, sb.len, 0);
 			strbuf_release(&sb);
+			o->found_changes = 1;
 			goto free_ab_and_return;
 		}
 		if (fill_mmfile(o->repo, &mf1, one) < 0 ||

t/t4017-diff-retval.sh

@@ -145,6 +145,14 @@ test_expect_success 'option errors are not confused by --exit-code' '
 
 for option in --exit-code --quiet
 do
+	test_expect_success "git diff $option returns 1 for changed binary file" "
+		test_when_finished 'rm -f .gitattributes' &&
+		git reset --hard &&
+		echo a binary >.gitattributes &&
+		echo 2 >>a &&
+		test_expect_code 1 git diff $option
+	"
+
 	test_expect_success "git diff $option returns 1 for copied file" "
 		git reset --hard &&
 		cp a copy &&