Open a mirrored NERDTree with correct width (#1177)

* Make sure a mirrored NERDTree is displayed at correct width.

* Remove references to unused variables.

b:NERDTreeOldWindowSize was referenced, but never set anywhere. No need
to keep it around.

* Refactor: Initialize variables a different way.

Using the get() function allows us to "let" variables more directly,
without using the execute command or requiring strings to be escaped.
This also eliminates the s:initVariable function. The new format is
shown below, and defines a default value if the user didn't provide a
value in the vimrc file.

  let g:var = get(g: 'var', 'default value')

* Update version number in change log.

* Fix alignment of `let g:var = get(g:, 'var', <default>)` statements.

lib/nerdtree/creator.vim

@@ -172,6 +172,7 @@ function! s:Creator.createMirror()
     let t:NERDTreeBufName = bufferName
     call self._createTreeWin()
     exec 'buffer ' .  bufferName
+    call b:NERDTree.ui.restoreScreenState()
     if !&hidden
         call b:NERDTree.render()
     endif

lib/nerdtree/opener.vim

@@ -174,9 +174,8 @@ function! s:Opener._newSplit()
 
     "resize the tree window if no other window was open before
     if onlyOneWin
-        let size = exists('b:NERDTreeOldWindowSize') ? b:NERDTreeOldWindowSize : g:NERDTreeWinSize
         call nerdtree#exec('wincmd p', 1)
-        call nerdtree#exec('silent '. splitMode .' resize '. size, 1)
+        call nerdtree#exec('silent '. splitMode .' resize '. g:NERDTreeWinSize, 1)
         call nerdtree#exec('wincmd p', 0)
     endif
 

lib/nerdtree/ui.vim

@@ -516,8 +516,7 @@ endfunction
 " zoom (maximize/minimize) the NERDTree window
 function! s:UI.toggleZoom()
     if exists('b:NERDTreeZoomed') && b:NERDTreeZoomed
-        let size = exists('b:NERDTreeOldWindowSize') ? b:NERDTreeOldWindowSize : g:NERDTreeWinSize
-        call nerdtree#exec('silent vertical resize '. size, 1)
+        call nerdtree#exec('silent vertical resize '. g:NERDTreeWinSize, 1)
         let b:NERDTreeZoomed = 0
     else
         call nerdtree#exec('vertical resize '. get(g:, 'NERDTreeWinSizeMax', ''), 1)