Modernize commit/edge transaction when parsing commit messages

Summary: Ref T5245. With work elsewhere (notably, D9839) we can remove this TODO and use real transactions.

Test Plan: Pushed a closes Txxx commit and got a close + transaction.

Reviewers: chad, btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5245

Differential Revision: https://secure.phabricator.com/D9848