[RFC] Add command to synchronize an imported repository

NOTE: This PR is not to be merged, but I just need to get some feedback.

This patch adds a command (that needs to be refined) to synchronize an imported repository with its origin.
It accepts also a different remote and source/dest branches (e.g., origin:branchA to local:branchB).

The main point missing (besides more tests) is that we need to trigger something like a GitPushService...