fast repo updates

[Mike McLean]

I posted this patch to rpm-metadata-list, but I figure some folks here 
might be interested as well.

http://people.redhat.com/mikem/software/createrepo-update2.patch

This patch allows createrepo to recycle existing metadata so that it 
only has to scan the rpms that have been changed or added. For 
situations where you might want to make small incremental changes to a 
yum repo (e.g. mock), this can provide significant time savings.