Unlike --no-pristine-tree, a --no-working-tree option would primarily
be useful for "crippled" repositories to which pushing/pulling is
allowed, but editing the working tree is not.  If there is no working
tree, it can't be edited accidentally!

In such a repo, "darcs apply" should be smart enough to not try to
edit the (non-existent) working tree.  Similarly "darcs whatsnew"
should abort with a meaningful warning, rather than claiming that the
entire working tree has been deleted.

Finally, AFAICT "darcs push" to a working-treeless repo would not
require "darcs apply" to be run on the remote end.  This would allow
push to servers that do not have a darcs binary installed (issue554).

Due to the changed semantics for apply, whatsnew, push, and probably
many other commands, I guess this change will require another entry in
_darcs/format.

PS: please don't call it "no-working-dir".

To be merged with issue431?