bug#20839: 25.0.50; file-in-directory-p doesn't understand windows backslashes

[Artur Malabarba]

On windows, the following form returns nil, but I think it should
return non-nil (\\ and / are usually accepted interchangeably on
windows).

    (file-in-directory-p "~/.emacs.d/elpa/magit/" "~\\.emacs.d\\elpa")

This is causing an issue[1] on the package.el's detection of external
packages on windows.

[1]: http://emacs.stackexchange.com/q/13166/50