diff-mode fixes for empty lines in unified diffs

[Stefan Monnier]

I've just installed on the 22 branch some changes to make it work better
in the face of unified diffs which have empty lines.  I've done a bit of
testing on it, but the change is non-trivial, so if you could all try to
break it, that would be great, thanks.


        Stefan


PS: Miles, you'll probably get merge conflicts on the trunk, just send
them to me.