Provide a low-level SVN commit query, and merge the VCS query types

Summary: Ref T4195. Even though we use `svnlook` in the hook itself, I need this query elsewhere, so provide it and merge the classes into one which does the right thing.

Test Plan:
  - Used `reparse.php` to reparse messages for Git, SVN and Mercurial commits, using `var_dump()` to examine the commit refs for sanity.
  - Used `reparse.php` to reparse changes for an SVN commit.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7800