allow local destinations with no remotes

[stavxyz]

Fails here: https://github.com/stavxyz/gitmux/blob/master/gitmux.sh#L314-L315

Requested features:

1. if destination directory exists and is not a git repository, fail with "[destination] is not a git repository"
2. if destination directory doesn't exist, create a new git repository there
3. if destination directory exists, is a git repository, but has no remote, don't fail (effectively, same behavior as (2))