pull should remove local branches pruned from remote if they do not contain diverging changes

[zombiezen]

Basically, the logic should be that if there was a remote tracking ref that matches the name of a local branch and that gets pruned during a pull, the local branch should be removed if there are no divergent commits.

[zombiezen]

Questions to be answered:

• Does this happen for any remote or just origin? If the latter, why? Perhaps this should check whether the branch is present in other remotes before pruning?
• When pruning, rather than wholly removing, perhaps we just move the branch to a refs/gg-old/BRANCH namespace. The user can periodically clean these out when they want to, but at least there's separation.