Add a sync flag that fetches only current branch

There is also shortcuts in case if the "current branch" is
a persistent revision such as tag or sha1. We check if the
persistent revision is present locally and if it does - do
no fetch anything from the server.

This greately reduces sync time and size of the on-disk repo

Change-Id: I23c6d95185474ed6e1a03c836a47f489953b99be