These days, we usually think of conflicts as one base state and series
of diffs between other states. The base state is normally the parent
when rebasing.
Also, we're deprecated `jj backout` in favor of `jj revert`, so let's
use that terminology.
We used to have documention about how conflicts are implemented, but I
removed that a long time ago when I rewrote the README to target users
rather than VCS hackers. Let's have a doc for the VCS hackers (and
curious users) as well, though.
