git.py: Improve mirror support

When implementing fetching from mirrors, I encountered some problems
with the git source:

1. The mirror URL was using translate_url()'s output, so if a different
   alias was used, then fetching from the mirror would go to a different
   directory, and be inaccessible.
2. After fixing that, fetching was unable to pull from a URL other than
   the one used at repository creation, meaning it wouldn't actually
   pull from the mirror.