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/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