Support content pre-commit hooks in Mercurial

Summary: Ref T4195. Add Mercurial support to the content hook phase.

Test Plan:
Here are some `commit` push logs for a Mercurial repo:

{F90689}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7792