Support SVN pre-commit hoooks

Summary:
Ref T4189. This adds SVN support, which was a little more messy than I though. Principally, we can not use PHABRICATOR_USER for Subversion, because it strips away the entire environment for "security reasons".

Instead, use --tunnel-user plus svnlook author to figure out the author.

Also fix "ssh://" clone URIs, which needs to be "svn+ssh://".

Test Plan:

• Made SVN commits through the hook.
• Made Git commits, too, to make sure I didn't break anything.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4189

Differential Revision: https://secure.phabricator.com/D7683