Add support for user edits to VC command arguments

Add support for user edits to VC command arguments

* lisp/vc/vc-dispatcher.el (vc-pre-command-functions): New hook.
(vc-want-edit-command-p): New variable.
(vc-do-command): If vc-want-edit-command-p is non-nil, prompt the user
to edit the VC command & arguments before execution.  Run the new hook.
(vc-do-async-command): Use the new hook to insert into BUFFER the
command that's next to be run.
* lisp/vc/vc-git.el (vc-git--pushpull): Drop prompting code.  Bind
vc-want-edit-command-p so that vc-do-command handles the prompting.
Use the new hook to update compile-command with the edited command.
* lisp/vc/vc.el (vc-print-branch-log): A non-nil prefix argument now
means vc-want-edit-command-p is bound to a non-nil value (bug#57807).