backwards-compatibility: support the post-indexchanged hook

When our patches to support that hook were upstreamed, the hook's name
was eliciting some reviewer suggestions, and it was renamed to
`post-index-change`. These patches (with the new name) made it into
v2.22.0.

However, VFSforGit users may very well have checkouts with that hook
installed under the original name.

To support this, let's just introduce a hack where we look a bit more
closely when we just failed to find the `post-index-change` hook, and
allow any `post-indexchanged` hook to run instead (if it exists).

hook.c

@@ -173,10 +173,22 @@ int run_hooks_opt(const char *hook_name, struct run_hooks_opt *options)
 		.hook_name = hook_name,
 		.options = options,
 	};
-	const char *const hook_path = find_hook(hook_name);
+	const char *hook_path = find_hook(hook_name);
 	int jobs = 1;
 	int ret = 0;
 
+	/*
+	 * Backwards compatibility hack in VFS for Git: when originally
+	 * introduced (and used!), it was called `post-indexchanged`, but this
+	 * name was changed during the review on the Git mailing list.
+	 *
+	 * Therefore, when the `post-index-change` hook is not found, let's
+	 * look for a hook with the old name (which would be found in case of
+	 * already-existing checkouts).
+	 */
+	if (!hook_path && !strcmp(hook_name, "post-index-change"))
+		hook_path = find_hook("post-indexchanged");
+
 	if (!options)
 		BUG("a struct run_hooks_opt must be provided to run_hooks");
 

t/t7113-post-index-change-hook.sh

@@ -16,6 +16,36 @@ test_expect_success 'setup' '
 	git commit -m "initial"
 '
 
+test_expect_success 'post-indexchanged' '
+	mkdir -p .git/hooks &&
+	test_when_finished "rm -f .git/hooks/post-indexchanged marker" &&
+	write_script .git/hooks/post-indexchanged <<-\EOF &&
+	: >marker
+	EOF
+
+	: make sure -changed is called if -change does not exist &&
+	test_when_finished "echo testing >dir1/file2.txt && git status" &&
+	echo changed >dir1/file2.txt &&
+	: force index to be dirty &&
+	test-tool chmtime -60 .git/index &&
+	git status &&
+	test_path_is_file marker &&
+
+	test_when_finished "rm -f .git/hooks/post-index-change marker2" &&
+	write_script .git/hooks/post-index-change <<-\EOF &&
+	: >marker2
+	EOF
+
+	: make sure -changed is not called if -change exists &&
+	rm -f marker marker2 &&
+	echo testing >dir1/file2.txt &&
+	: force index to be dirty &&
+	test-tool chmtime -60 .git/index &&
+	git status &&
+	test_path_is_missing marker &&
+	test_path_is_file marker2
+'
+
 test_expect_success 'test status, add, commit, others trigger hook without flags set' '
 	test_hook post-index-change <<-\EOF &&
 		if test "$1" -eq 1; then