The tunnel mode ra_svn data stream can be corrupted in two ways:
* Dotfiles can output to stdout before svnserve runs.
* Hook scripts can output to stdout, adding text to the middle of
the data stream.
When this happens, the user sees a cryptic "Malformed network data"
We can solve the second problem by closing stdout before invoking hook
scripts, although doing this well may involve non-portable gook
We can solve the first problem by introducing a guard at the beginning
of the protocol (either just in tunnel mode or all the time); that is,
the server would send something like "\n---svn datastream starts
here---\n" before sending text. Or, we can introduce a guard for
every communication ("svn:<length>:<data>" for every chunk of text
sent, either just in tunnel mode or all the time).