R1:618b5cbbc4e8

Install pre-commit hooks in Git repositories

Summary:
Ref T4189. T4189 describes most of the intent here:

- When updating hosted repositories, sync a pre-commit hook into them instead of doing a `git fetch`.
- The hook calls into Phabricator. The acting Phabricator user is sent via PHABRICATOR_USER in the environment. The active repository is sent via CLI.
- The hook doesn't do anything useful yet; it just veifies basic parameters, does a little parsing, and exits 0 to allow the commit.

Test Plan…