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