git: add 'refspec' argument

This argument may be used to fetch additional refs beyond the default refs/heads/* and refs/tags/*.
For example, useful to check out a GitHub pull request or Gerrit patch set.
Ssorry it took so long to review this PR.  This will either be 1.8.3 or 1.9 -- Not sure whether we'll have a 1.8.3 yet.