Parse shorthand refspecs as valid

Relax the ONELEVEL ref naming rules so the refspec parsing code can
ask for 'master' to be considered valid.
remote: dwim the refspecs according to the remote's advertised refs

As git allows you to store shorthand refspecs in the configuration, we
need to do this ourselves.
Merge pull request #1511 from carlosmn/refspec-shorthand

dwim shorthand refspecs for fetch