This is a very nice new feature suggested by Joachim. It could be quite
easily implemented by a new Darcs hacker.

When you have multiple repositories that you interact with, darcs should
offer a menu (maybe if you pass in --ask-repos) that lets you select
which one.  Right now, the way to do it is to cat _darcs/prefs/repos and
pass it to darcs with --no-set-defaults

Note that in the Darcs community, we have this mechanism called the
Grumpy Old Man process which we use to actively resist new features, so
if you do implement this, be prepared to be challenged! See
http://wiki.darcs.net/Ideas :-D

Note that this could reuse some implementation work in patch308 for 
issue1530