It actually decreases readability of the .do files - by not making it
explicit when you're going into a subdir.
Plus it adds ambiguity: what if there's a dirname.do *and* a dirname/all?
We could resolve the ambiguity if we wanted, but that adds more code, while
taking out this special case makes *less* code and improves readability.
I think it's the right way to go.
Normally, creating the target $1 yourself is bad; create $3 instead. But if
$1 is a directory, we'll allow it. That way 'redo subdir' can call
subdir.do, and subdir.do can both create the directory *and* run a bunch of
sub-.do files on it.
