relnotes: various tweaks for release notes
Stop filtering out commits that are expected to be covered by releasenotes
extension: now we want two lists, one for WhatsNew and one for ReleaseX.Y.
Use only(stoprev, startrev) to make relnotes -h output be actually true
about what revisions are included.
More filter rules, mostly obvious.
More classifying rules to have less things in "unsorted".
Looks like nargs=1 was just making args.startrev and args.stoprev be lists for
BC and API sections are renamed to what we're using on the WhatsNew page, and
also just skipped if empty.