Local-only workflow

[schacon]

Currently the client only works for a workflow where there is a remote repository and assumes that upstream integration merges happen on that server.

If a repository doesn't have a remote, we can't even add it properly.

A simple way to allow people to use GB local only, we can simply allow you to choose a local branch as your target branch. Then instead of "push" we could have "merge" that just merges it into the local target branch (and updates the base).