R1:f93c6985ad84

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

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