fold: clarify message for empty revision set and abort instead (BC)
I find it clearer that there are no revisions to fold because none
were specified. Otherwise it seems ambiguous: maybe there are no
revisions to fold because it's impossible to fold whatever was given?
In analogy of other commands that are given no revisions (e.g. strip,
graft), this is also now an abort instead of a mere error message,
which is a BC.