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:

After:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7776

Differential Revision: https://secure.phabricator.com/D12730