fix Log keyword expansion with trailing whitespace in prefix

Our expansion of the Log keyword was slightly wrong. We need to
trim trailing whitespace from the "prefix" line content which
preceeds the Log keyword when we write out line content which
followed the Log keyword. Update the Log expansion example given
in a comment to document this (see there for details; this behaviour
of CVS is hard to explain without illustration).

Found while testing conversion of the OpenBSD CVS repository.
Add a new test which uses an RCS file from this repository to
reproduce this problem.