Support Mercurial pretxnchangegroup hooks

Summary: Ref T4189. Fixes T2066. Mercurial has a lot of hooks so I'm not 100% sure this is all we need to install (we may need separate hooks for tags/bookmarks) but it should cover most of what we're after at least.

Test Plan:

• bin/repository pull'd a Mercurial repo and got a hook install.
• Pushed to a Mercurial repository over SSH and HTTP, with good/bad hooks. Saw hooks fire.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2066, T4189

Differential Revision: https://secure.phabricator.com/D7685