R1:e9be9ecfbcc9

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