New vc-async-checkin user option

New vc-async-checkin user option

* lisp/vc/vc.el (vc-async-checkin): New option.
(vc-checkin): Don't use with-vc-properties on or display
messages around asynchronous checkins.
* lisp/vc/vc-git.el (vc-git-checkin):
* lisp/vc/vc-hg.el (vc-hg-checkin, vc-hg-checkin-patch): Perform
an async checkin operation when vc-async-checkin is non-nil.
* doc/emacs/vc1-xtra.texi (General VC Options):
* etc/NEWS: Document the new option.

* lisp/vc/vc-dispatcher.el (vc-wait-for-process-before-save):
New function.
(vc-set-async-update): If the current buffer visits a file, call
vc-refresh-state.
* lisp/vc/vc-hg.el (vc-wait-for-process-before-save): Autoload.
(vc-hg--async-command, vc-hg--async-buffer, vc-hg--command-1):
New utilities, partially factored out of vc-hg-command.
(vc-hg-merge-branch): Use vc-hg--async-command, thereby newly
respecting vc-hg-global-switches.

(cherry picked from commit 8e02537d0be3cfdeaaf7764e2ef2db8b66de542a)