update nerdtree's statusline

add let g:airline#extensions#nerdtree_statusline's documentation

autoload/airline/extensions.vim

@@ -28,7 +28,6 @@ let s:filetype_overrides = {
       \ 'gundo': [ 'Gundo', '' ],
       \ 'help':  [ 'Help', '%f' ],
       \ 'minibufexpl': [ 'MiniBufExplorer', '' ],
-      \ 'nerdtree': [ get(g:, 'NERDTreeStatusline', 'NERD'), '' ],
       \ 'startify': [ 'startify', '' ],
       \ 'vim-plug': [ 'Plugins', '' ],
       \ 'vimfiler': [ 'vimfiler', '%{vimfiler#get_status_string()}' ],
@@ -43,6 +42,12 @@ if exists(':Gina') && (v:version > 704 || (v:version == 704 && has("patch1898"))
   let s:filetype_overrides['gina-tag'] = ['gina', '%{gina#component#repo#preset()}' ]
 endif
 
+if get(g:, 'airline#extensions#nerdtree_statusline', 1)
+  let s:filetype_overrides['nerdtree'] = [ get(g:, 'NERDTreeStatusline', 'NERD'), '' ]
+else
+  let s:filetype_overrides['nerdtree'] = ['NERDtree', '']
+endif
+
 let s:filetype_regex_overrides = {}
 
 function! s:check_defined_section(name)

doc/airline.txt

@@ -785,6 +785,10 @@ Airline displays the Nerdtree specific statusline (which can be configured
 using the |'NerdTreeStatusline'| variable (for details, see the help of
 NerdTree)
 
+* enable/disable nerdtree's statusline integration >
+  let g:airline#extensions#nerdtree_status = 1
+<  default: 1
+
 -------------------------------------                      *airline-nrrwrgn*
 NrrwRgn <https://github.com/chrisbra/NrrwRgn>