fix a bug when doing :NERDTreeMirror without 'hidden set

doc/NERD_tree.txt

@@ -952,6 +952,7 @@ The latest dev versions are on github
 
 3.0.1
     - fix a bug where :NERDTreeToggle was broken unless 'hidden was set
+    - fix a bug with :NERDTreeMirror when 'hidden wasnt set
     - make the q mapping work with secondary (:e <dir>  style) nerd trees
     - more insane refactoring and doc updates
 

plugin/NERD_tree.vim

@@ -2163,6 +2163,9 @@ function! s:initNerdTreeMirror()
     let t:NERDTreeBufName = bufferName
     call s:createTreeWin()
     exec 'buffer ' .  bufferName
+    if !&hidden
+        call s:renderView()
+    endif
 endfunction
 " FUNCTION: s:tabpagevar(tabnr, var) {{{2
 function! s:tabpagevar(tabnr, var)