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