Revision 1461823

On the fsfs-format7 branch:  Allow svn_stringbuf_ts to be only
temporarily converted to svn_string_t, i.e. don't modify them.
This is fine since svn_string_ts are considered immutable.

* subversion/libsvn_subr/string.c
  (svn_stringbuf__morph_into_string): remove paranoia debug code