Get the path of the shared common directory for this repository.
If the repository is bare, it is the root directory for the repository. If the repository is a worktree, it is the parent repo's gitdir. Otherwise, it is the gitdir.
A repository object
the path to the common dir
See Implementation
Get the path of the shared common directory for this repository.
If the repository is bare, it is the root directory for the repository. If the repository is a worktree, it is the parent repo's gitdir. Otherwise, it is the gitdir.