* Force sort to recalculate the cached sortKey.
The problem in issue #880 was caused by the sort using the old sortKey.
For example, given nodes A, B, and C, if B were renamed to D, the sort
was still using B as its sortKey, thus not moving it.
It's a bit of a hack, but if we set g:NERDTreeOldSortOrder to an empty
list, the cached sortKey will be recalculated. I did the same thing for
both the Copy and Add functions as well.
* Add a comment to explain the let ... = [] statement.
f98078d3ae