{cvs,svn}import: use the new 'git read-tree --empty'

Since fb1bb96 (read-tree: deprecate syntax without tree-ish args,
2010-09-10) not passing --empty caused a spurious warning that was
shown to the user.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>