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 no reason. BC and API sections are renamed to what we're using on the WhatsNew page, and also just skipped if empty.

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