R1:2f6784ee1c41

Add workflow to create repository identities

Summary:
Depends on D19443. Creates a workflow for populating the new identity table by iterating over commits, either one repo at a time or all at once. Locally caches identities to avoid fetching them `inf` times. An actual migration that invokes this workflow will come in another revision that won't land until at least next week.

Performance is ~2k commits in 4.9s on my local machine.

Test Plan: Ran locally a few times with a few different states of the…