vim: fix filename of undotree.vim
vim: fix filename of undotree.vim

The extension matters here, but only on Vim without `:packadd`.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent