Add CGR function (change to Git root)

This function uses `git rev-parse` to CD to the root directory in a git
repo.
Update quick move (qm.fish) for more accuracy

Update qm.fish's handling of the `ls` data to strip any '@' signs
(signifying a symbolic link). Also remove any extra debug output.
Fix for brand new git repos

Add error handling for brand new git repos. Whereas previously a new git
repo would cause an error message to be displayed every time the prompt
was printed (due to there being no branches in the current repo), now
the function silently ignores these errors.