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
Support Mercurial pretxnchangegroup hooks

Summary: Ref T4189. Fixes T2066. Mercurial has a //lot// of hooks so I'm not 100% sure this is all we need to install (we may need separate hooks for tags/bookmarks) but it should cover most of what we're after at least.

Test Plan:
  - `bin/repository pull`'d a Mercurial repo and got a hook install.
  - Pushed to a Mercurial repository over SSH and HTTP, with good/bad hooks. Saw hooks fire.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2066, T4189

Differential Revision: https://secure.phabricator.com/D7685