[PATCH] psvn: Fix bogus comparisons of buffers to strings.

The current code is obviously wrong, and this is the right fix
(at least until psvn is changed to support multiple svn-status
buffers, in which case the major mode should be checked instead).
However, I am not sure the code *should* be fixed: I don't
understand its purpose, it has apparently never worked, and
nobody has complained, so perhaps it should be just removed
instead.