R1:95eab2f3b0f2

Record parent relationships when discovering commits

Summary:
Ref T4455. This adds a `repository_parents` table which stores `<childCommitID, parentCommitID>` relationships.

For new commits, it is populated when commits are discovered.

For older commits, there's a `bin/repository parents` script to rebuild the data.

Right now, there's no UI suggestion that you should run the script. I haven't come up with a super clean way to do this, and this table will only improve performance for now, so it's not…