The file name darcs creates automatically for bundles when we send
(or obliterate) more than one patch is currently based on the name
of the first patch (in history/application order). There are many
situations where it would be easier to recognize the bundle if it
was instead based on the last patch we send. This is especially true
when sending a patch causes dependencies to be added to the bundle
that are not (yet) in the remote repo.
Perhaps we should change the default here?