Fix fatal in Git hook when a --force push completely rewrites a ref

Summary: Fixes T4224. If you git merge-base A B, and they have no ancestor, the command exits with an error. Assume errors mean "no ancestry" and continue.

Test Plan: Completely rewrite a repository with a --force push.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4224

Differential Revision: https://secure.phabricator.com/D7756