Allow branch deletions to be pushed in Mercurial

Summary: Fixes T5050. This might not be 100% right in all edge cases, but it worked on everything I tried.

Test Plan:
  - Pushed a branch deletion.
  - Pushed a branch creation.
  - Pushed a brnach creation + deletion.
  - Pushed a brnach deletion + creation.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5050

Differential Revision: https://secure.phabricator.com/D9122