We can negate a `darcs add` with `darcs remove` and vice versa without
changing the working tree. We can try to negate a `darcs replace old new`
with `darcs replace new old`, but
(1) that changes the working tree,
(2) it may fail to cancel the previous replace if a manual edit afterwards
has added a line containing 'old'.
The command would convert a pending replace patch to a sequence of hunks
with the same effect when applied.
Alternatively, we could offer a more general command similar to `git reset -
-mixed` that allows to remove and kind of changes from pending (without
changing the working tree).