remove NERDTreeDirArrows option

Use +/~ for windows - which seems to not have the arrow chars in its
default font. TBH I don't really understand this.

Inprove the UI indent matching so that it should handle any combo of
open/close symbol lengths e.g. the fancy arrows are 3 bytes each,
whereas +/~ are 1 byte each.

doc/NERD_tree.txt

@@ -671,9 +671,6 @@ NERD tree. These options should be set in your vimrc.
 |'NERDTreeMinimalUI'|           Disables display of the 'Bookmarks' label and 
                                 'Press ? for help' text.
 
-|'NERDTreeDirArrows'|           Tells the NERD tree to use arrows instead of
-                                + ~ chars when displaying directories.
-
 |'NERDTreeCascadeOpenSingleChildDir'|
                                 Cascade open while selected directory has only
                                 one child that also is a directory.
@@ -986,19 +983,6 @@ of the following lines to set this option: >
     let NERDTreeMinimalUI=1
 <
 
-------------------------------------------------------------------------------
-                                                           *'NERDTreeDirArrows'*
-Values: 0 or 1
-Default: 0.
-
-This option is used to change the default look of directory nodes displayed in
-the tree. When set to 0 it shows old-school bars (|), + and ~ chars. If set to
-1 it shows right and down arrows.  Use one of the follow lines to set this
-option: >
-    let NERDTreeDirArrows=0
-    let NERDTreeDirArrows=1
-<
-
 ------------------------------------------------------------------------------
                                           *'NERDTreeCascadeOpenSingleChildDir'*
 Values: 0 or 1

lib/nerdtree/ui.vim

@@ -287,13 +287,11 @@ endfunction
 
 "FUNCTION: s:UI._indentLevelFor(line) {{{1
 function! s:UI._indentLevelFor(line)
-    let level = match(a:line, '[^ \-+~'.g:NERDTreeDirArrowExpandable.g:NERDTreeDirArrowCollapsible.'`|]') / s:UI.IndentWid()
-    " check if line includes arrows
-    if match(a:line, '['.g:NERDTreeDirArrowExpandable.g:NERDTreeDirArrowCollapsible.']') > -1
-        " decrement level as arrow uses 3 ascii chars
-        let level = level - 1
-    endif
-    return level
+    "have to do this work around because match() returns bytes, not chars
+    let numLeadBytes = match(a:line, '\M\[^ '.g:NERDTreeDirArrowExpandable.g:NERDTreeDirArrowCollapsible.']')
+    let leadChars = strchars(a:line[0:numLeadBytes-1])
+
+    return leadChars / s:UI.IndentWid()
 endfunction
 
 "FUNCTION: s:UI.IndentWid() {{{1

plugin/NERD_tree.vim

@@ -65,9 +65,14 @@ call s:initVariable("g:NERDTreeShowFiles", 1)
 call s:initVariable("g:NERDTreeShowHidden", 0)
 call s:initVariable("g:NERDTreeShowLineNumbers", 0)
 call s:initVariable("g:NERDTreeSortDirs", 1)
-call s:initVariable("g:NERDTreeDirArrows", !nerdtree#runningWindows())
-call s:initVariable("g:NERDTreeDirArrowExpandable", "▸")
-call s:initVariable("g:NERDTreeDirArrowCollapsible", "▾")
+
+if !nerdtree#runningWindows()
+    call s:initVariable("g:NERDTreeDirArrowExpandable", "▸")
+    call s:initVariable("g:NERDTreeDirArrowCollapsible", "▾")
+else
+    call s:initVariable("g:NERDTreeDirArrowExpandable", "+")
+    call s:initVariable("g:NERDTreeDirArrowCollapsible", "~")
+endif
 call s:initVariable("g:NERDTreeCascadeOpenSingleChildDir", 1)
 
 if !exists("g:NERDTreeSortOrder")