worktree: validate worktree paths

Worktree paths need to fix within MAX_PATH always, regardless of
`core.longpaths` setting.

src/worktree.c

@@ -135,6 +135,9 @@ static int open_worktree_dir(git_worktree **out, const char *parent, const char
 		goto out;
 	}
 
+	if ((error = git_path_validate_workdir(NULL, dir)) < 0)
+		goto out;
+
 	if ((wt = git__calloc(1, sizeof(*wt))) == NULL) {
 		error = -1;
 		goto out;