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

[Nikolay Kudryavtsev <nikolay.kudryavtsev@gmail.com>]

Ok, I was actually wrong in my last message, that file with commit 
message should always be in the same encoding as git commitencoding. 
There's no weirdness.

> 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. 
That's why I kept blaming git.
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.

> Am I missing something?
No, I just provided a practical example of:

> The 2nd solution is somewhat slower, but it is better, because it will
> allow to write log messages using any characters, not just those
> representable in the current codepage


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

-- 
Best Regards,
Nikolay Kudryavtsev