Use stable commit identifier to load repository commit
Summary: Fixes T5113. This was caught in the crossfire of cleaning up the DiffusionRequest "commit" properties.
Test Plan: Loaded /rXnnnn with some of the nnn missing.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T5113
Differential Revision: https://secure.phabricator.com/D9253