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