Merge branch 'mt/open-worktree'

Clean up the code that checks if a directory is a Git repo. Use git
rev-parse instead of rolling our own logic to find that out. A side
effect (which also happens to be the main motivation behind it) of this
change is that git-gui can now open worktrees other than the main
worktree.

* mt/open-worktree:
  git-gui: allow opening work trees from the startup dialog