R1:973b8ace8616

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