bug#23595: 25.1.50; file with chinese/japanse chars, vc-diff fails (HG, Git, RCS)

[Eli Zaretskii]

> Cc: oub@mat.ucm.es, eggert@cs.ucla.edu, 23595@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Tue, 24 May 2016 12:35:40 +0300
> 
> >What Emacs should do is
> > bind coding-system-for-read to utf-8 in this case (not leave it
> > unbound as in your patch), under the assumption that the user used the
> > procedure outlined by Paul.
> 
> Should `utf-8' altogether replace `undecided' in 
> vc-coding-system-for-diff? Then the use of buffer-file-coding-system 
> could be predicated on its being compatible with ascii.

Not sure it's a good idea: the solution we found is only known to work
with Git, whereas vc-coding-system-for-diff is for any VCS.  Mercurial
seems to have a similar encode/decode filter feature, but I'm not sure
using it means the diff results will be in UTF-8.

I think we should have a git-specific function that implements the
above idea, and then we should use it in vc-coding-system-for-diff.
(I prefer a separate function because my gut feeling is that we will
need something like that in other Git operations, when UTF-16 files
are involved.)

WDYT?