cola: allow working in a bare repository

cola: allow working in a bare repository

Support working in a bare repository by avoiding worktree paths.
There are a few places where we want the current directory,
and were doing path operations with the result of
Git.worktree(), which returns None in a bare repository.

Add Git.getcwd() so that callers can ask for the current directory
in a consistent way without needing to care about the worktree
vs. bare repository distinction.

For regular repositories, Git.getcwd() returns the worktree,
which allows it to be used in place of Git.worktree().

For bare repositories, Git.getcwd() returns the gitdir.
The dual behavior allows callers to easily handle bare repositories.

Update callers to use Git.getcwd() in the fsmonitor, main model,
and app initialization.

Signed-off-by: David Aguilar <davvid@gmail.com>