docs: add concepts guide (the set theory behind obsolescence)

docs: add guide to sharing mutable history

docs: add new user guide This has also been reviewed to death on evolve-testers. There are still a couple of short sections to write (clearly marked "TODO"), and one example to add. But (if I may be so bold) this is a gigantic improvement over the current docs, so it really should get merged. Incidentally, the figures are all SVG files created with Inkscape. They're not perfect, but they're pretty nice. Anyone who knows a better way to create technical diagrams is welcome to contribute. One wart: there's a test script that accompanies the document and largely duplicates it, but I haven't unified them. I've been concentrating on writing the best possible content, not on fiddling with tools. I suspect that unifying them will be non-trivial, but definitely worth doing.

docs: rewrite index page (including dangling links to future docs) This has been reviewed to death on the evolve-testers mailing list. I've made some minor tweaks since the last round of reviews, but the main content is the same.

amend: add -D/--current-date option, just like mq's qrefresh has Carefully designed so we can easily add -D to other changeset-creating commands (probably anything that takes -d), and so that we can add -U/--current-user as well. When that's done, the date and user options should be very similar to those in MQ.

amend: explicitly test -d option