bug#23076: 24.5; vc-git: add a new variable for log output coding system

[Eli Zaretskii <eliz@gnu.org>]

> From: Nikolay Kudryavtsev <nikolay.kudryavtsev@gmail.com>
> Cc: 23076@debbugs.gnu.org
> Date: Sat, 9 Apr 2016 17:14:07 +0300
> 
> > How can it break someone's setup, if using the current system codepage
> > is the_only_  way of invoking programs from Emacs on MS-Windows?
> Download this:
> https://github.com/msysgit/msysgit/releases/download/Git-1.7.9-preview20120201/PortableGit-1.7.9-preview20120201.7z
> Then, from emacs -Q:
> (setq vc-git-program "d:/Downloads/PortableGit/bin/git.exe")
> Note that the git is used from bin, and not from cmd. Russian commits do 
> work with this version, when vc-git-commits-coding system is utf-8. 

This cannot work except by accident, and/or for only some specific
characters.  Windows attempts to translate non-ASCII characters passed
to programs to the current codepage, so a byte that is not possible in
the codepage will be mangled.  For example, try a log message that
uses a UTF-8 string which has a 0x98 byte in it -- the only byte that
is not a valid cp1251 character -- you will see it mangled on the Git
side.

> So we can define breaking someone's setup - if that someone uses this 
> old git version, forcing windows-1251 would break his setup.
> 1.7.9 is the last working version.

Even if I'm wrong about this, v1.7.9 is too old to care about.  It's
too easy to upgrade.

> In the end, of course I like solution #2.

Thanks, I will work on implementing it.