bug#28883: w32-convert-standard-filename

[Tak Kunihiro]

Path returned from `convert-standard-filename' of 26.0.90 differs from
one of 25.2.  Is this bug?

;;; GNU Emacs 26.0.90 (build 3, x86_64-w64-mingw32) of 2017-10-14
emacs -Q 
(convert-standard-filename (expand-file-name "~/.emacs.d")) ; => 
"c:/Users/dream/.emacs.d"

;;; GNU Emacs 25.2.1 (x86_64-w64-mingw32) of 2017-04-25
emacs -Q
(convert-standard-filename (expand-file-name "~/.emacs.d")) ; => 
"c:\\Users\\dream\\.emacs.d"