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 repository_identity table.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: jcox, Korvin, PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D19446