Only call `realpath` on `Pathname` object.

lib/cocoapods/project.rb

@@ -179,13 +179,13 @@ module Pod
     # @return [PBXFileReference] The new file reference.
     #
     def add_file_reference(absolute_path, group, reflect_file_system_structure = false)
-      file_path_name = Pathname.new(absolute_path)
+      file_path_name = absolute_path.is_a?(Pathname) ? absolute_path : Pathname.new(absolute_path)
       group = group_for_path_in_group(file_path_name, group, reflect_file_system_structure)
 
-      if ref = reference_for_path(absolute_path.realpath)
+      if ref = reference_for_path(file_path_name.realpath)
         ref
       else
-        ref = group.new_file(absolute_path.realpath)
+        ref = group.new_file(file_path_name.realpath)
         @refs_by_absolute_path[absolute_path.to_s] = ref
       end
     end