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