R1:74251b363675

Support bookmark hook operations in Mercurial

Summary: Ref T4195. Turns bookmark mutations in Mercurial into log objects.

Test Plan:
Pushed a pile of bookmarks and got logs:

{F89313}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7764