submodule.c: stricter checking for submodules in is_submodule_modified

By having a stricter check in the superproject we catch errors earlier,
instead of spawning a child process to tell us.

Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -1052,11 +1052,12 @@ unsigned is_submodule_modified(const char *path, int ignore_untracked)
 	git_dir = read_gitfile(buf.buf);
 	if (!git_dir)
 		git_dir = buf.buf;
-	if (!is_directory(git_dir)) {
+	if (!is_git_directory(git_dir)) {
+		if (is_directory(git_dir))
+			die(_("'%s' not recognized as a git repository"), git_dir);
 		strbuf_release(&buf);
 		/* The submodule is not checked out, so it is not modified */
 		return 0;
-
 	}
 	strbuf_reset(&buf);