add option 'NERDTreeAlwaysShowUpDirLine=0'

The option "NERDTreeMinimalUI" is used to remove some non functional parts of the nerdtree ui, not only disables display of the 'Bookmarks' label and 'Press ? for help' text, but also the 'up a dir' line is removed. So add this option "NERDTreeAlwaysShowUpDirLine" to make the 'up a dir' line always display if they want.