bug#29189: 25.3; Dired does not work with binary filenames

[Eli Zaretskii]

> From: Allen Li <vianchielfaura@gmail.com>
> Date: Fri, 1 Dec 2017 21:21:14 -0800
> Cc: Andreas Schwab <schwab@suse.de>, handa@gnu.org, 29189@debbugs.gnu.org
> 
> I applied this patch to master (0b6f4f2c60) and it fixes the bug and
> doesn't crash Emacs immediately.  The code also looks right, but I am
> not familiar with Emacs's C code.  A few questions.
> 
> Why do we have to handle multibyte strings in this function
> decode_coding?  (I found the answer in the docs)

decode_coding needs to work when a series of raw bytes is inserted
into a multibyte buffer (which happens in the Dired case).

> Can you briefly explain how Emacs internally stores unibyte and
> multibyte strings?  (I found the answer in character.h)

Right, the details are in that header file.

> After doing the above research, I can more confidently say this is
> right, but having an expert opinion would be nice.

Thanks for proofreading the patch and testing it.  I'm waiting for
Handa-san to comment on it.