R1:85f505465e53

Support serving SVN repositories over SSH

Summary:
Ref T2230. The SVN protocol has a sensible protocol format with a good spec here:

http://svn.apache.org/repos/asf/subversion/trunk/subversion/libsvn_ra_svn/protocol

Particularly, compare this statement to the clown show that is the Mercurial wire protocol:

> It is possible to parse an item without knowing its type in advance.

WHAT A REASONABLE STATEMENT TO BE ABLE TO MAKE ABOUT A WIRE PROTOCOL

Although it makes substantially more sense than Mercurial…