DownloadPatchSet: fetch the change only, and nothing else.

* Previously, it would run `git fetch <remote.name> <change refspec>
  <remote.fetch>, which would fetch all the branches, even if 'sync-c'
  was set to true in the manifest.
* Fix that, since all it needs to fetch is the change that was asked
  for, and nothing else.
* For some more info, refer to the discussion on:
  I42a9d419b51f5da03f20a640ea68993cda4b6500

Change-Id: Ibc801695d56fc16e56f999e0f61393f54461785f
1 file changed