cvsimport: use git-update-index --index-info

This should reduce the number of git-update-index forks required per
commit. We now do adds/removes in one call, and we are no longer forced to
deal with argv limitations.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>