more detailed message for #assert_date_sorted

utilities.rb

@@ -2,8 +2,11 @@
 module RepoDepot
   module InstanceMethods
     def assert_date_sorted events
-      events_to_check = events.select {|e| e.date }
-      raise "events out of date order" unless events_to_check.each_cons(2).map {|e1, e2| e1.date < e2.date }.all?
+      transitions = events.select {|e| e.date }.each_cons(2).to_a
+      return unless transitions.map { |e1, e2| e1.date > e2.date }.any?
+
+      failure = transitions.detect { |e1, e2| e1.date > e2.date }
+      raise "events out of order #{failure[0].date}, #{failure[1].date}"
     end
   end
 end