cvs2hg fixed to work with modern Mercurials

[Marcin Kasperski]

I fixed some incompatibilities due to which cvs2hg wasn't working with
Mercurials >= 3.1. My fixed version is published at

   https://bitbucket.org/Mekk/mercurial-cvs2hg

(I offered it upstream but I am not sure whether original author still
maintains the tool)

The changed version should still work with older Mercurials too (the
code attempts to discover which API is appropriate).