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 symbolic branches, using @{branch name}. For
the mainline branch in CVS, we use @{trunk}. This enables repository map
configurations like:

    cvs.@{trunk} = master
    git.master = @{trunk}

Replaces #12 and #17
Fixes #11