bug#35049: 25.1; ibuffer-do-sort-by-filename does not sort all buffers

[Eli Zaretskii <eliz@gnu.org>]

> From: Axel Svensson <mail@axelsvensson.com>
> Date: Sat, 30 Mar 2019 13:36:08 +0000
> 
> To reproduce:
>  1) In a shell, run: mkdir ~/dir-a ~/dir-b ~/dir-c
>  2) In a shell, run: touch ~/dir-a/filea1 ~/dir-c/filec1 ~/dir-c/filc2
>  3) find-file ~/a and enter 20 characters in this buffer.
>  4) find-file ~/c and enter 30 characters in this buffer.
>  5) find-file ~/b and enter 10 characters in this buffer.
>  6) find-file ~/dir-a
>  7) find-file ~/dir-c
>  8) find-file ~/dir-b
>  9) M-x ibuffer. You should see three Fundamental buffers named a, b,
>     and c, as well as three "Dired by name" buffers named dir-a, dir-b
>     and dir-c.
> 10) Press s s
> 11) The modeline says "IBuffer by size", and the buffers are ordered by
>     size, i.e. b,a,c,dir-b,dir-a,dir-c.
> 12) Press s f
> 13) The modeline says "IBuffer by filename/process", but not all the
>     buffers are ordered. The Fundamental buffers are ordered a,b,c
>     correctly, but the Dired buffers are ordered dir-b,dir-a,dir-c as
>     before pressing s f.

Dired buffers have the buffer-file-name as nil, which is why their
order is not changed by "s f".