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).