Improve handling of non-ASCII characters in Git log messages

Improve handling of non-ASCII characters in Git log messages

* lisp/vc/vc-git.el (vc-git-commits-coding-system): Now a defcustom.
(vc-git-log-output-coding-system): New defcustom.
(vc-git-print-log, vc-git-command, vc-git--call): Use
'vc-git-log-output-coding-system' for reading stuff from Git.
Don't override values of 'coding-system-for-read/write' if they
are bound by caller -- this allows the user to force an encoding
via "C-x RET c".
(vc-git-checkin): On MS-Windows, pass the log message via a
temporary file, to work around the limitations on passing
non-ASCII characters via command-line arguments.  Force using the
'locale-coding-system' for Git command-line arguments.  This fixes
problems with non-ASCII commit log messages on MS-Windows.
(Bug#23076)

* etc/NEWS: Mention the new vc-git related defcustoms.