cave/trend_tag: limit before window to 3 * period

cave/src/trend_tag.rs

@@ -89,16 +89,17 @@ impl TrendTag {
         }
 
         let from = until - period;
+        let not_before = from - 3 * period;
         let mut before_mentions = 0;
         let mut before_hours = 0;
         let mut after_mentions = 0;
 
         for (hour, mentions) in self.by_hour.iter().cloned() {
-            if hour <= from {
+            if hour > from {
+                after_mentions += mentions;
+            } else if hour > not_before {
                 before_mentions += mentions;
                 before_hours += 1;
-            } else {
-                after_mentions += mentions;
             }
         }