posts: fix incorrect post counts for -pool:, -fav: searches

Fix `-pool:1234` and `-fav:evazion` searches incorrectly returning the
same post count as `pool:1234` and `fav:evazion` searches.

app/logical/post_query.rb

@@ -111,12 +111,12 @@ class PostQuery
     ast.none?
   end
 
-  # True if the search is a single metatag search for the given metatag.
+  # True if the search is a single, non-negated metatag search for the given metatag. Assumes the query has been normalized.
   def is_metatag?(name, value = nil)
     if value.nil?
-      is_single_term? && has_metatag?(name)
+      metatag? && has_metatag?(name)
     else
-      is_single_term? && find_metatag(name) == value.to_s
+      metatag? && find_metatag(name) == value.to_s
     end
   end