that reverts the doc diff.

This changes mark-new-version to create a backmerge branch that will
additionally handle the revert of the doc diff that's now created for
every release.

Sidebar: I really wish I knew a good git visualizer for GitHub - this
thing is going to start creating some awesome Gordian knots of merges.
(Or more like little Omegas, since they basically just change
version/base.go.)

Fixes #10825