cherry-pick, undo, and rollback UI

There are several different interfaces to cherry-picking and rollback, and
I'm sort of on the fence about which one(s) it's best to support.

For cherry-pick, the obvious interface is 'apply the diff from version X
to version Y to my local version'. There's also 'pull everything just from
file F to my local version' (we've been calling this 'selective update',
and it's reasonably easy to support without supporting full-blown
cherry-pick, but is a form of cherry-picking nonetheless). Finally,
there's the interface where you say 'pull in everything which is new in X
as conflict markers', then you locally pick and choose which changes to
accept, then do a checkpoint, and it gets treated as a cherry-pick.

For rollback, there's a lot more subtlety and nuance. The basic
functionality is 'take the diff from version X to version Y, undo it from
my local version, and let it get re-done when I merge with a descendant of
version Z'.  Undo simply leaves off the version Z. Note that Z frequently
is pointing out to a version which hasn't been updated from locally. I
think a good modification of this approach is 'take all modifications
which were made by a descendant of X and an ancestor of Y, undo them
locally, and redo them when I merge with a descendant of Z'. That does a
good job of avoiding undoing unrelated changes which just happened to get
checked in. Another interface is to do it cherry-picking style, and update
everything which is different in version X as conflict markers, then
manually pick out what to remove, checkpoint, and have it treated as a
rollback, again to be undone next time one merges with version Z.

It also may make sense to not really have a specified version Z, and
instead have the system warn the user when there was a conflict with a
rollback, and have an easy way of saying 'well then, undo the rollback'.

Obviously rollback has the least nice interface here, although honestly I
think that anybody stuck needing rollback will be so desperate for
something which works they won't complain about interface all that much.

-Bram