Load commits affected by revert language in Diffusion message parser

Summary:
Ref T1751. This still doesn't do anything very interesting, but loads the acutal Commit objects that a commit message claims to revert.

The only tricky thing here is that we need to interpret "reverts rnnn" or "reverts nnn" in an SVN repository as "reverts rXnnn", where "X" is the current repository. This adds a method to do allow `DiffusionCommitQuery` to do that.

Test Plan:
Used `reparse.php --message` to reparse several commits with revert language and verify they loaded the correct affected commits.

In an SVN repository, created a commit with ambiguous revert language ("reverts n", "reverts rn", "reverts n, n") and verified it identified the affected commits correctly despite ambiguity.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1751

Differential Revision: https://secure.phabricator.com/D5842