No longer tolerate a multitude of .changes files when doing push.

No longer tolerate a multitude of .changes files when doing push.

Instead, insist on a single one.  This eliminates a potential for a
variety of surprises, where dgit would upload something different to
the user expected.

We no longer need to behave this way because nowadays the sbuild build
(which was the real use case, because it would generate a .source and
a .ARCH changes as well as the final .multi) renames the intermediate
changes files.

Also --rm-old-changes helps.