Write a basic SSH pull log for Git

Summary: Ref T11766. When users run git pull or similar, log the operation in the pull log.

Test Plan: Performed SSH pulls, got a log in the database. Today, this event log is purely diagnostic and has no UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11766

Differential Revision: https://secure.phabricator.com/D16738