Allow Herald rules to check for revisions with no reviewers

Summary: Fixes T4225. Adds the NON_EXISTS condition to Herald for "Reviewers", and adds a few more conditions which have reasonable meanings.

Test Plan: Used test console to check a revision with reviewers, and another without reviewers. Both produced the expected results.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4225

Differential Revision: https://secure.phabricator.com/D7757
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