cvsimport: use git-update-ref when updating

This simplifies code, and also fixes a subtle bug: when importing in a
shared repository, where another user last imported from CVS, cvsimport
used to complain that it could not open <branch> for update.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>