R1:78aab6d4a56f

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…