fix(worktree): align with Git's gitdir resolution

Git only normalizes relative references in `gitdir`[1], so do the same.

[1]: https://github.com/git/git/blob/v2.54.0/setup.c#L1012-L1025

Author: elovelan

git/repo/base.py

@@ -295,7 +295,7 @@ def __init__(
                 sm_gitpath = find_worktree_git_dir(dotgit)
 
             if sm_gitpath is not None:
-                git_dir = expand_path(sm_gitpath, expand_vars)
+                git_dir = osp.normpath(sm_gitpath)
                 self._working_tree_dir = curpath
                 break