always cvs export -D now

For very large exports in active CVS repositories, a "cvs export"
command becomes more and more likely to see only part of a commit
if a commit covers files in multiple directories being committed
during the export process, leading to hysteresis.

Previously, "-D now" was used for a consistent export timestamp
only for CVS trunk.  Now it is used for all "cvs export" operations.

Fixes #23