Story: use newest created_at of all merged stories for calculating hotness

we already take their points, but if they have a much newer
created_at and we've been moved off the front page, points won't
save us.  use their created_at to be fresher.

should fix #300

app/models/story.rb

@@ -206,8 +206,15 @@ class Story < ActiveRecord::Base
       sign = 0
     end
 
-    return -((order * sign) + base +
-      ((self.created_at || Time.now).to_f / HOTNESS_WINDOW)).round(7)
+    # if any newer stories were merged into us, use the newest created_at to
+    # take its place on the front page
+    tcreated_at = (self.created_at || Time.now).to_f
+    if (mtca = self.merged_stories.map{|ts| ts.created_at.to_f }.max) &&
+    mtca > tcreated_at
+      tcreated_at = mtca
+    end
+
+    return -((order * sign) + base + (tcreated_at / HOTNESS_WINDOW)).round(7)
   end
 
   def can_be_seen_by_user?(user)