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:

• Performed Git pushes and pulls over SSH and HTTP.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4189

Differential Revision: https://secure.phabricator.com/D7682