Move VC diff ASYNC argument to the fifth position

Move VC diff ASYNC argument to the fifth position

* lisp/vc/vc-svn.el (vc-svn-diff):
* lisp/vc/vc-src.el (vc-src-diff):
* lisp/vc/vc-sccs.el (vc-sccs-diff):
* lisp/vc/vc-rcs.el (vc-rcs-diff):
* lisp/vc/vc-mtn.el (vc-mtn-diff):
* lisp/vc/vc-hg.el (vc-hg-diff):
* lisp/vc/vc-git.el (vc-git-diff):
* lisp/vc/vc-dav.el (vc-dav-diff):
* lisp/vc/vc-cvs.el (vc-cvs-diff):
* lisp/vc/vc-bzr.el (vc-bzr-diff):
* lisp/obsolete/vc-arch.el (vc-arch-diff): Move ASYNC argument to the end.

* lisp/vc/vc.el (vc-diff-internal): Pass `async' argument to the
backend `diff' command in the last position.