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

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

Hello Eli.

Just to explain the underlying issue.

With emacs -Q try committing to the same repository by copy-pasting the 
previous commit message. Then do git log from shell. Your commit message 
would get broken.

This happens because git on Windows expects the commit message to be in 
your Windows "language for non-Unicode programs" encoding. Then it 
recodes from it to utf-8.

So, to be able to commit in russian we:
1. Change language for non-Unicode programs to russian.
2. (setq vc-git-commits-coding-system 'windows-1251)

After doing this, commiting in russian would work. But now our C-x v l 
is broken. We can either fix it by setting logoutputencoding in git, but 
this would break git log outside of emacs, or add a new variable to vc, 
and that's what I want.

That's a relatively recent change in git, from 2013 or 2014, so if 
you're using some really old version, everything might just work out of box.

-- 
Best Regards,
Nikolay Kudryavtsev