Improve performance of generating synthetic changesets
Summary: Ref T7776. This could get better, but I think I got most of the big stuff. It's ~4x faster now.
Test Plan:
Before:
{F393338}
After:
{F393339}
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T7776
Differential Revision: https://secure.phabricator.com/D12730