add CVS commit ID support to rlog.py

Newer CVS clients tag commits with a commit ID which allows us to
correctly convert commits which changed several RCS files at once.
The rsync access method based on cvs2gitdump was already taking
advantage of this. To ensure that conversions over the pserver
protocol yield the same result as conversions over rsync we need
to add commit ID support to rlog.py.

Add two new test cases which convert the same repository over
rsync and pserver respectively, and ensure that they yield the
same result. Without commit ID support conversion over pserver
produces a different result for this particular test repository.

With feedback about coding style from vlorentz.