bug#29149: Tramp shell uses local shell setting in windows

[Eli Zaretskii]

> From: Shuguang Sun <shuguang@gmail.com>
> Date: Thu, 9 Nov 2017 17:27:51 +0800
> 
> Local: Emacs on Windows 7.
> Host:  Linux
> Tramp from Windows 7 to linux
> 
> A further information: it is about backup. When I setup 
> tramp-backup-directory-alist which will make it backup
> serverside. However, It concat !driver_c to the backup file name.
> 
> I have setting ("\\.md$" . "~/.emacs.d/backup-tramp/mdfiles") in 
> tramp-backup-directory-alist. The backup file
> in the server is 
> "!drive_c!plink!!user@host!!!opt!host!path!Ava!_summary.md.~1~"
> 
> The "!drive_c" comes very strange.

Michael, any advice on this one?  Do you understand where did
!drive_c! come from?  AFAIU, it should only be used for backups of
local files.