Bugfix: git fetch directly from the remote URL, rather than only setting a default the first time

- Fixes problem where the gitian URL varies for different branches of the same repository
- Explicitly fetching tags using + allows repository to overwrite existing tags (changed tag)
- Fetching branch heads to local heads correctly makes branch names available for commit option
- Fetching both branches and tags at the same time optimizes things slightly
Merge pull request #29 from luke-jr/bugfix_changing_remote_uri

Bugfix: git fetch directly from the remote URL, rather than only setting a default the first time