Add a "branches" rule for commits

Summary:
Fixes T1353. Also some minor unrelated cleanup:

  - `openTransaction()` / `saveTransaction()` exist now, fix TODOs.
  - Fix some instructions.
  - Make `diffusion.branchquery` return empty for SVN rather than fataling.

Test Plan:
  - Added a branches rule.
  - Ran a dry run against commits in different VCSes.

{F105574}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, Nopik

Maniphest Tasks: T1353

Differential Revision: https://secure.phabricator.com/D8086