Do the heavy lifting for git commit hooks

Summary:
Ref T4189. This doesn't add any rules yet, but does all the heavy lifting to figure out what's changed and put it in a consuamble (if somewhat ad-hoc) datastructure, which lists all the ref and tag modifications and all the new commits in a consistent way.

From here, it should be fairly straightforward to add top-level rules (e.g., ff pushes only).

Test Plan: Output is huge, see comments.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4189

Differential Revision: https://secure.phabricator.com/D7687