abbreviate-file-name on Windows seems incorrect

[Drew Adams]

On MS Windows, my $HOME is "c:\\". That is what is returned by (getenv
"HOME"), and that is what I see in Windows itself, in the list of
environment vars.

The code for `abbreviate-file-name' defines `abbreviated-home-dir' as
follows, in order to be able to "substitute `~' for the user's home
directory", as the doc string says:

(or abbreviated-home-dir
    (setq abbreviated-home-dir
          (let ((abbreviated-home-dir "$foo"))
            (concat "^" (abbreviate-file-name
                         (expand-file-name "~"))
                    "\\(/\\|\\'\\)"))))

The comment for this code is as follows, which indicates that a slash is
added to distinguish the home dir from a file in that dir:

    ;; We include a slash at the end, to avoid spurious matches
    ;; such as `/usr/foobar' when the home dir is `/usr/foo'.

However, that is improper for a Windows home directory such as mine (not an
uncommon value).  (expand-file-name "~") returns "c:/", which is correct,
but the result for `abbreviated-home-dir' becomes "^c:/\\(/\\|\\'\\)", which
is incorrect and useless, AFAICT. It should be something like
"^c:\\(/\\|\\'\\)", I would think.

I would expect (abbreviate-file-name "c:/foo/bar") to return "~/foo/bar",
but it returns "c:/foo/bar". If `abbreviated-home-dir' were
"^c:\\(/\\|\\'\\)", then the result would be correct: "~/foo/bar".

I'm not sure exactly what the resulting regexp should be or how it should be
constructed.  "^c:\\(/\\|\\'\\)" works for "c:/foo/bar" (returning
"~/foo/bar"), but not for "c:/" (it returns "c:/", not "~").

The point is that I don't see how the current regexp could ever work for a
Windows $HOME that starts with a drive letter. This seems like a bug, to me.
This behavior is the same in Emacs 20, 21, and 22, but I think it is wrong.

Since `abbreviate-file-name' is used a lot, I imagine that the code that
uses it must be compensating for this bug somehow when it comes to Windows,
or else there are other, dependent bugs lurking elsewhere. (?)

I want to be able to do, for instance, (abbreviate-file-name
(expand-file-name "~/WHATEVER")), and have it work normally on Windows,
returning "~/WHATEVER". It currently does not - it returns "c:/WHATEVER"
instead. It should also work for empty WHATEVER, of course (i.e. "~/" and
"~").