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 important that we get everyone to run the script right away. I'm just leaving it for the moment, and we can figure out how to tell admins to run it later.

The ultimate goal is to solve T2683, but solving T4455 gets us some stuff anyway (for example, we can serve diffusion.commitparentsquery faster out of this cache).

Test Plan:

• Used bin/repository discover to discover new commits in Git, SVN and Mercurial repositories.
• Used bin/repository parents to rebuild Git and Mercurial repositories (SVN repos just exit with a message).
• Verified that the table appears to be sensible.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: jhurwitz, epriestley

Maniphest Tasks: T4455

Differential Revision: https://secure.phabricator.com/D9044