feat: support bare repo

[rockyzhang24]

Closes #84

Solution:

Change git -C to git --git-dir /path/to/gitdir --work-tree /path/to/worktree. gitdir is the directory containing .git or the value of $GIT_DIR env. worktree is the value of $GIT_WORK_TREE env or the parent directory of gitdir.

I've been using git bare repo to manage my dot files for years and it works very well in my bare repo.

Test:

...

Thank you.