When observing a repository in Git, just "fetch <url>" without worrying about the "origin" remote

Summary:
Depends on D20420. Ref T13277. We currently spend substantial effort trying to detect and correct the URL of the "origin" remote in Git repositories.

I believe this is unnecessary, and we can always git fetch <url> ... to get the desired result instead of git muck-with-origin + git fetch origin .... We already do this in the more recent parts of the codebase (e.g., intracluster sync) and it works correctly in every case I'm aware of.

Test Plan:

• Grepped for origin, origin .
• Ran bin/repository update ... to fetch a mirrored repository.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13277

Differential Revision: https://secure.phabricator.com/D20421