Remove dependence on callsigns from bin/commit-hook

Summary:
Ref T4245. Two effects:

• First, let hooks work for future repositories without callsigns.
• Second, provide a better error when users push directly to hosted repositories.

Test Plan: Ran bin/commit-hook PHID-REPO-xxx.

Reviewers: chad, avivey

Reviewed By: avivey

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D15293