New get-change-comment VC backend action

New get-change-comment VC backend action

This gets us closer to using log-view-modify-change-comment with modern
VCS.  What remains is implementing the modify-change-comment backend
action for those VCS.

* lisp/vc/vc.el: New get-change-comment backend action.
(vc-modify-change-comment): Pass the backend to vc-start-logentry.
* lisp/vc/log-view.el (log-view-extract-comment): Use new
get-change-comment action.
* lisp/vc/vc-git.el (vc-git-get-change-comment): Factor out of
vc-git-log-edit-toggle-amend.

(cherry picked from commit 01c8f31a54df511913ad88cb6138a01390faafbe)