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.

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Migrated from D6623 (view on Phabricator)