bug#26911: 25.2; eshell "cd .." doesn't work correctly with TRAMP

[Paul Eggert]

> > I installed the attached patch to revert the recent expand-file-changes in the
> > DOS_NT case, which should fix the problem you mentioned.
>
> Thanks, it does.  But it produces a different problem:
>
>    (expand-file-name "." "c:/foo/bar/") => "c:/foo/bar
>
> (note the absence of the trailing slash).

That's what Emacs 27 does on MS-Windows, no? So it's not a regression, and the 
problem can be fixed at the convenience of whoever's interested in hacking on 
the MS-Windows side of the code.

Another way to put it is that Bug#26911 is now fixed for GNU and POSIX, but not 
for MS-Windows. My earlier changes attempted to fix it for all platforms, but 
this had undesirable side-effects in MS-Windows so I withdrew the MS-Windows 
part of the changes. I have therefore reopened Bug#26911 since I assume it's 
still present on MS-Windows.

Are some of the new test cases failing on MS-Windows? Should I arrange for these 
test cases to be expected to fail on MS-Windows? If so, please let me know which 
ones are failing, so that I can do that.

> I'm not interested in messing with expand-file-name

That's understandable as expand-file-name is quite a mess internally. But if 
you're not interested in any attempt to clean up the mess, I guess I should 
refrain from giving it a shot.