Fix new NERDTrees' width when previous one was in the only window. (#1153)

* Fix new NERDTrees' width when previous one was in the only window.

When leaving a NERDTree buffer, its window's width is remembered so that
that width can be used when showing the buffer again in a new window. If
NERDTree is the only window when leaving the buffer, it remembers the
whole width of the editor. When a new NERDTree window is created, it is
sized such that there is only a very small window for files to be opened
into.

To fix this, if NERDTree is the ONLY window, remember its width as the
value of g:NERDTreeWinSize, not the width of the editor.

* Update version number in change log.

CHANGELOG.md

@@ -5,6 +5,7 @@
         - **.PATCH**: Pull Request Title (PR Author) [PR Number](Link to PR)
 -->
 #### 6.9
+- **.3**: Fix new NERDTrees' width when previous one was in the only window. (PhilRunninger) [#1153](https://github.com/preservim/nerdtree/pull/1153)
 - **.2**: Fix the scope of several key mappings (lifecrisis, PhilRunninger) [#1151](https://github.com/preservim/nerdtree/pull/1151)
 - **.0**: Enable opening bookmarks in split windows. (PhilRunninger) [#1144](https://github.com/preservim/nerdtree/pull/1144)
 #### 6.8

lib/nerdtree/ui.vim

@@ -368,7 +368,7 @@ function! s:UI.saveScreenState()
         call g:NERDTree.CursorToTreeWin()
         let self._screenState['oldPos'] = getpos('.')
         let self._screenState['oldTopLine'] = line('w0')
-        let self._screenState['oldWindowSize']= winwidth('')
+        let self._screenState['oldWindowSize'] = winnr('$')==1 ? g:NERDTreeWinSize : winwidth('')
         call nerdtree#exec(win . 'wincmd w', 1)
     catch
     endtry