Is there a way to to update svn packages only when there are updates in the svn source? Actually I use a script to grep "9999" from eix -I output and put all those packages in a package set, then I have a cron job to rebuild this set weekly. What I'd like to do is to only fetch the svn sources, and rebuild the packages if sources have been updated since last build, saving lots of compilations. Does portage (or any other tool) provide a way to do it?