Add Wiki#size to efficiently count pages without loading them.

lib/gollum/wiki.rb

@@ -210,6 +210,17 @@ module Gollum
       tree_list(treeish || 'master')
     end
 
+    # Fill an array with a list of pages.
+    #
+    # ref - A String ref that is either a commit SHA or references one.
+    #
+    # Returns a flat Array of Gollum::Page instances.
+    def size(ref = nil)
+      tree_map_for(ref || 'master').inject(0) do |num, entry|
+        num + (@page_class.valid_page_name?(entry.name) ? 1 : 0)
+      end
+    end
+
     # Public: All of the versions that have touched the Page.
     #
     # options - The options Hash: