Move an is_a check + cast to Ruby

Author: tenderlove

ext/rugged/rugged_tree.c

@@ -489,9 +489,6 @@ static VALUE rb_git_tree_diff_(VALUE self, VALUE rb_repo, VALUE rb_self, VALUE r
 		Data_Get_Struct(rb_self, git_tree, tree);
 	}
 
-		if (TYPE(rb_other) == T_STRING)
-			rb_other = rugged_object_rev_parse(rb_repo, rb_other, 1);
-
 		if (rb_obj_is_kind_of(rb_other, rb_cRuggedCommit)) {
 			git_tree *other_tree;
 			git_commit *commit;

lib/rugged/tree.rb

@@ -12,6 +12,10 @@ def self.diff(repo, _self, other = nil, options = {})
           diff_tree_to_tree repo, _self, options
         end
       else
+        if other.is_a?(::String)
+          other = Rugged::Object.rev_parse repo, other
+        end
+
         _diff(repo, _self, other, options)
       end
     end