R1:e48c36697ad4

Make blame UI recover gracefully if Identities haven't been built yet for a commit

Summary:
See PHI1014. We may not have Identities if you race the import pipeline, or in some other cases which are more "bug" / "missing migration"-flavored.

Load the commit data so we can fall back to it if we don't have identities.

Test Plan:
- Wiped out all my identities with `UPDATE ... SET authorIdentityPHID = NULL WHERE ...`.
- Before change: blame fataled with `Attempting to access attached data on…