update patch, take 3

http://people.redhat.com/mikem/software/createrepo-update5.patch

This patch adds a --update option to createrepo. If that option is 
specified and the directory provided has preexisting repodata, then that 
data is read and compared to the present file list. Only RPMs that have 
been added or changed (detected by change in mtime or size) are scanned; 
the rest of the repodata is recycled.

When the amount of change is slight, this greatly reduces the amount of 
IO for the run. Unchanged files get by with just an os.lstat. In my 
testing this is at least twice as fast as using --cachedir.

In a system like Koji, where repositories are recreated with minor 
updates on a regular basis, these savings can really add up. Please take 
this patch, so that we don't have to choose between performance and 
upstream compatibility.