CVS import: Support new-style .cvspass-file format.
Based on an email from Ralf.Leibold at nuance dot com
Update on pull -u when heads were closed (fixes
issue666).
Patch written by Benoit Boissinot.
This should probably be improved in the future to handle long-living
branches, as joining two "other" heads will switch to that new head.
But this is not a new problem, as adding to the "other" head would have
switched to that new head, too.