Use correct encoding to communicate with Git.

 lisp/vc/vc-git.el (vc-git-command, vc-git--call): Bind
 coding-system-for-read and coding-system-for-write to
 vc-git-commits-coding-system.
Use "~1" instead of "^" in vc-git.el to specify the parent of a Git commit.

 lisp/vc/vc-git.el (vc-git-previous-revision): Use "~1" instead of
 "^", since the latter is a special character for MS-Windows system
 shells.