Log Message:
Update to 45:
do not treat a failed rcscheckout() as system failure

A failed checkout is not necessarily caused by a solid system error, but
can be also caused by a faulty RCS file. Do not use rb_sys_fail(), since
it will abort() if errno == 0. Instead, simply raise a RuntimeError.

Reported-by: Brad King

check deltas for correctness before applying

We can not blindly trust the RCS file to contain correct deltas. We have
to verify that the chunks (especially deletes) make sense and operate on
existing lines.

Reported-by: Brad King