git ssb

As mentioned by @mixmix, it would be useful to be able to merge a pull request from the web UI.

A difficult part would be to detect if a PR can be merged cleanly. However a start would be to detect if it can be fast-forward merged, which is the easy case.

This may entail adding a feature to pull-git-repo or ssb-git-repo for repo merging or for a "can merge" check