[RFC] Add command to sync an imported repository

NOTE: This PR is not to be merged, but I just need to get some feedback.

This patch adds support to keep in sync an imported repository with a remote one.
It's not complete, but I need to get some feedback to understand if it'll ever make into master... :wink: 
It's tightly related to this PR: https://github.com/gitlabhq/gitlab-shell/pull/95

Note that it's just a proof of concept and there are several points missing.
Most notably:
* the underlying structure can sync different remotes and different source/dest branches, but the current UI will just synchronize origin and master:master
* tests are missing
* it's unfinished and may eat your pets...