sync-all get should respect branches
If I have ghc-7.8 checked out and I do a './sync-all get', I might expect the repositories that are retrieved would also be set for ghc-7.8. However, this does not currently appear to be the case.
This may be a little trickier, since if you're on a topic branch but you are mostly based off of master, then 'sync-all get' will fail, since the topic branch in question won't exist. So we should probably fall back in some way, rather than fail if the specified branch is not available.
At the very least, we should print a warning if we sync-all get, the main branch is not master but we checked out a copy of master in the subrepo.