Make the revision kind in SVN++ svn::revision explicitly immutable.
It's illogical to be able to even theoretically change the kind of a revision object, so we make that restriction explicit by making the class member const.
[in subversion/bindings/cxx] * include/svnxx/revision.hpp: Include <new> for placement-new. (revision::tag): Make this immutable. (revision::operator=): New; work around the kind tag's immutability by destroying the object and creating a new object in place. * tests/test_revision.cpp (assignment): New test case.