Deal with rred shortcomings around empty patch files

The last commit doesn't really belong here, but I found it while debugging the rest and it deserves to be fixed even if the failure mode is as silent as for the rest as automatic fallback will deal with it making the overall action eventually successful.

The rred patches itself should be included in stable so that dak generates correct patches again after arriving at an empty patch, which would magically fix also unpatched apt clients (at least in terms of the "ignored as fallback activates" error message which spawned this hunt).

The renaming is not really important from a user pov and being in client-side merge codepaths also not in very active use (within Debian), but it stabilizes our tests and makes failures actual be reported as failures in the tests should we end up breaking acquire code in the future which is very important.

So, a bit of a stretch for all of them, but I would prefer them all being included in stable. I don't see much regression potential for either of them.