Pull is_a check to Ruby

Author: tenderlove

ext/rugged/rugged_index.c

@@ -810,15 +810,7 @@ static VALUE rb_git_index_diff(VALUE self, VALUE rb_other, VALUE rb_options)
 		// the "old file" side of the diff.
 		opts.flags ^= GIT_DIFF_REVERSE;
 
-		if (rb_obj_is_kind_of(rb_other, rb_cRuggedCommit)) {
-			git_tree *other_tree;
-			git_commit *commit;
-			Data_Get_Struct(rb_other, git_commit, commit);
-			error = git_commit_tree(&other_tree, commit);
-
-			if (!error)
-				error = git_diff_tree_to_index(&diff, repo, other_tree, index, &opts);
-		} else if (rb_obj_is_kind_of(rb_other, rb_cRuggedTree)) {
+		if (rb_obj_is_kind_of(rb_other, rb_cRuggedTree)) {
 			git_tree *other_tree;
 			Data_Get_Struct(rb_other, git_tree, other_tree);
 			error = git_diff_tree_to_index(&diff, repo, other_tree, index, &opts);

lib/rugged/index.rb

@@ -8,7 +8,11 @@ def diff(*args)
       if other.nil?
         diff_index_to_workdir options
       else
-        _diff other, options
+        if other.is_a? ::Rugged::Commit
+          _diff other.tree, options
+        else
+          _diff other, options
+        end
       end
     end