Generalize & simplify implementation of user edits to VC commands

Generalize & simplify implementation of user edits to VC commands

* lisp/vc/vc-dispatcher.el (vc-pre-command-functions)
(vc-want-edit-command-p): Delete.
(vc-filter-command-function): New variable.
(vc-user-edit-command): Factor out of vc-do-command.
(vc-do-command, vc-do-async-command)
* lisp/vc/vc-git.el (vc-git--pushpull)
* lisp/vc/vc.el (vc-print-branch-log): Use vc-filter-command-function
in place of vc-pre-command-functions and vc-want-edit-command-p.