Improve performance and remove race conditions during commit

Improve performance and remove race conditions during commit

Remove calls to reset() and dispose() after walking revisions or trees.
If they are no longer needed the calls will mainly cause more memory
to be allocated.

Create the RefUpdate object before making the commit so that there
will not be a later race condition when the branch reference is updated.

Thanks to Shawn O. Spearce for the advices.