Fix naive path concatenation in svn_path().

svn_path() was joining paths using simple string concatenation, which
is not always valid as some characters must be escaped. Thanks to Justin
Pitts for reporting this bug.
Sanity check SVN_REPO at startup.

Bad things can result if SVN_REPO does not point to the root of the
Subversion repository. Sanity check that it does point to the root on
startup to avoid potential confusion.