R1:017d6ccd0793

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.