contrib/synthrepo: return None to delete files on commit, don't raise IOError
The internal commit API was changed in
650b5b6e75ed to expect None from the
filectx function when a file is to be deleted, not an IOError. This change
keeps synthrepo up-to-date.
dockerrpm: create a yum/dnf repo from the generated rpms
This gives "PPA" functionality where users easily can stay uptodate with latest
nightly build.