See also issue2567 which fixed (restores) the correct behavior for
whatsnew, unless used interactively. All other commands that support the
option display hunks only in interactive mode, which is definitely
broken (tested with darcs-2.10.2).

The point to note here is that the functionality is inherently stateful:
each hunk changes the context of later hunks in the same file.

Fixing this is not completely trivial. The interactive selection process
for prim patches must keep track of the state, presumably by working in
a VirtualTreeIO monad.

We need a way to test output in interactive mode. I have no idea how to
do that ATM.

We should also rename the option (--unified, WTH?). I propose
--with-context or --show-context.

I am half of a mind to disable (i.e. temporarily remove) the option for
most commands except for whatsnew in non-interactive mode until this
issue is fixed.