bug#4623: 23.1.50; rmail changes encoding of characters on save

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: 4623@emacsbugs.donarmstrong.com
> Date: Sun, 06 Dec 2009 23:34:55 -0500
> 
> >   . When replying to a message, rmail-reply uses it to set the
> >     encoding of the *mail* buffer where you compose the reply, which
> >     takes care of 99.99% of situations where the response needs to
> >     have some non-trivial encoding that is not your system's native
> >     one.
> 
> rmail-replay doesn't need to use buffer-file-coding-system for that, but
> could just as well use some other variable for it, right?

If that variable is set correctly, sure.  However,
buffer-file-coding-system has the advantage that it is already
prominently shown in the mode line.

> > Maybe we need to set up an after-save-hook to restore the original
> > encoding after saving the message collection?
> 
> That would be fine, yes.  I'd suggest to use an rmail-coding-system
> variable as the canonical place to store the coding system used for the
> currently shown message, use it in rmail-reply in preference to
> buffer-file-coding-system, and simply copy rmail-coding-system to
> buffer-file-coding-system whenever necessary, such as in
> after-save-hook.

Well, I did precisely that, as you could see, but then Markus pointed
out that a single global variable will not work when there are more
than one Rmail buffer.

I will post a slightly improved patch soon that hopefully will take
care of that as well.