Add support for import/export of cvs mainline

CVS treats the mainline branch as a special case, insofar as the
commands to work with it do not support using the '-r' flag. This change
adds special handling for configs that reference 'cvs.MAIN' as a indicator
to use the mainline branch.

Includes lowlevel tests for importing main trunk from cvs and exporting
from git to main trunk in cvs.

Replaces pull request #12
Fixes #11