[fix] explicit remote and refspecs

  This commit works around the issue of older versions of Git not
properly recording remotes and refspecs by specifying them explicitly
during fetch.

  Test plan:
  - createtwo releases that differ in their Git gem dependency
    revisions -- for example, take
    - commit 7cf589 "Migrate Ruby cache dirs" and
    - <hack the cache to trim the objects ...>
    - commit 180771 "Assets precompilation in Rails 3 and fixes"
    on Ubuntu 10.04 (git version 1.7.0.4) and verify that the second
    release creation fails when fetching git objects
  - Apply this patch and create release again, verify that it works

Change-Id: Ifc8e18494e126fd51c189d9ff9bebf04989887b6
Merge "[fix] explicit remote and refspecs"