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

project.py

@@ -1236,7 +1236,6 @@ class Project(object):
     cmd = ['fetch', remote.name]
     cmd.append('refs/changes/%2.2d/%d/%d' \
                % (change_id % 100, change_id, patch_id))
-    cmd.extend(list(map(str, remote.fetch)))
     if GitCommand(self, cmd, bare=True).Wait() != 0:
       return None
     return DownloadedChange(self,