Remove diffusion.getcommits Conduit API method

Summary:
Ref T4245. This was obsoleted long ago and has no callers in Phabricator or Arcanist.

Also some minor cleanup.

Test Plan: grep for callers everywhere.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14930