Add a basic pull event log for debugging repository cloning

Summary:
Ref T10228. This is currently quite limited:

• No UI.
• No SSH support.

My primary goal is to debug the issue in T10228. In the long run we can expand this to be a bit fancier.

Test Plan:
Made various valid and invalid clones, got sucess responses and not-so-successful responses, viewed the log table for general corresponding messages and broad sanity.

Ran GC via bin/phd debug trigger, no issues.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10228

Differential Revision: https://secure.phabricator.com/D15127