fix for swap files and opening in a split

when opening a file in a new split (with the <tab> map) we were doing
the :split with a "silent" modifier. This meant that a swap existed and
input was required from the user, the script would just freeze up

doc/NERD_tree.txt

@@ -889,6 +889,7 @@ fridge for later ;)
       settings for each new NERD tree.
     - fix to window resizing when opening a file when NERD tree is the only
       window open
+    - other fixes
 
 2.11.0
     - changes to the 'o' mapping when opening files:

plugin/NERD_tree.vim

@@ -2320,7 +2320,7 @@ function! s:OpenNodeSplit(treenode)
 
     " Open the new window
     try
-        exec("silent " . splitMode." sp " . a:treenode.path.StrForEditCmd())
+        exec(splitMode." sp " . a:treenode.path.StrForEditCmd())
     catch /^Vim\%((\a\+)\)\=:E37/
         call s:PutCursorInTreeWin()
         throw "NERDTree.view.FileOpen exception: ". a:treenode.path.Str(0) ." is already open and modified."