Merge branch 'jk/diff-external-with-no-index' into maint-2.43

"git diff --no-index file1 file2" segfaulted while invoking the
external diff driver, which has been corrected.

* jk/diff-external-with-no-index:
  diff: handle NULL meta-info when spawning external diff

diff.c

@@ -4384,7 +4384,8 @@ static void run_external_diff(const char *pgm,
 		add_external_diff_name(o->repo, &cmd.args, two);
 		if (other) {
 			strvec_push(&cmd.args, other);
-			strvec_push(&cmd.args, xfrm_msg);
+			if (xfrm_msg)
+				strvec_push(&cmd.args, xfrm_msg);
 		}
 	}
 

t/t4053-diff-no-index.sh

@@ -205,6 +205,18 @@ test_expect_success POSIXPERM,SYMLINKS 'diff --no-index normalizes: mode not lik
 	test_cmp expected actual
 '
 
+test_expect_success POSIXPERM 'external diff with mode-only change' '
+	echo content >not-executable &&
+	echo content >executable &&
+	chmod +x executable &&
+	echo executable executable $(test_oid zero) 100755 \
+		not-executable $(test_oid zero) 100644 not-executable \
+		>expect &&
+	test_expect_code 1 git -c diff.external=echo diff \
+		--no-index executable not-executable >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success "diff --no-index treats '-' as stdin" '
 	cat >expect <<-EOF &&
 	diff --git a/- b/a/1