Add a new command vc-pull-and-push

Add a new command vc-pull-and-push

* lisp/vc/vc-svn.el (vc-exec-after):
* lisp/vc/vc-hg.el (vc-exec-after):
* lisp/vc/vc-git.el (vc-exec-after):
* lisp/vc/vc-cvs.el (vc-exec-after):
* lisp/vc/vc-bzr.el (vc-exec-after):
* lisp/org/org-macro.el (vc-exec-after):
* lisp/obsolete/vc-mtn.el (vc-exec-after):
* lisp/obsolete/vc-arch.el (vc-exec-after): Update declaration.
* lisp/vc/vc-dispatcher.el (vc--process-sentinel): Allow running
code only on success.
(vc-exec-after): Ditto.
(vc--inhibit-change-window-start): New variable.
(vc-do-async-command): Use it to allow chaining commands without
moving window point.  Return the process instead of the buffer,
since the process may have exited already, and then we can't get
at the process.

* lisp/vc/vc-git.el (vc-git--pushpull): Return the process object.
(vc-git-pull-and-push): New function.

* lisp/vc/vc.el (vc-pull-and-push): New command (bug#51964).