Omit "--" in older fallback commands for Git remote URIs

Summary: Ref T13481. Some older versions of Git appear to not support "--" in these commands. Just drop it. This can lead to ambiguous results with certain obviously-silly remote names, but doesn't appear to lead to anything dangerous.

Test Plan: Will followup with user on ancient Git.

Maniphest Tasks: T13481

Differential Revision: https://secure.phabricator.com/D20952