R1:75c359835903

Require commit identities when editing commits to resolve an issue with audit actions not applying properly

Summary:
See <https://discourse.phabricator-community.org/t/cannot-audit-a-git-commit/2848>. In D20581, I made some audit behavior dependent upon identities, but the actual edit flow doesn't load them. This can cause us to raise an "attach identities first" exception in the bowels of the edit workflow and trigger unexpected behavior at top level.

Load identities when editing a commit so that the…