Properly update refs so reset works

Calling `git fetch origin master` will update the FETCH_HEAD but not refs/remotes/origin/master so the subsequent `git reset --hard origin/master` does not reset to the fetched head.  Calling `git fetch origin` (without the refspec) updates both FETCH_HEAD and refs/remotes/origin/master.  So the following hard reset does what is expected.
Merge pull request #30 from tschaub/fetch

Properly update refs so reset works.