If a file's already open in the window, don't edit it again. (#1103)

* If a file's already open in the window, don't edit it again.

* Update version number in change log.

* Make only one call to self.str().

lib/nerdtree/path.vim

@@ -295,7 +295,10 @@ endfunction
 
 " FUNCTION: Path.edit() {{{1
 function! s:Path.edit()
-    exec 'edit ' . self.str({'format': 'Edit'})
+    let l:bufname = self.str({'format': 'Edit'})
+    if bufname() !=# l:bufname
+        exec 'edit ' . l:bufname
+    endif
 endfunction
 
 " FUNCTION: Path.extractDriveLetter(fullpath) {{{1