Support git commit --amend/--signoff

Support git commit --amend/--signoff

* lisp/vc/log-edit.el (log-edit-font-lock-keywords): Allow hyphens in
header names.
(log-edit-toggle-header): New function.
(log-edit-extract-headers): Accept function values in HEADERS alist.

* lisp/vc/vc-git.el (vc-git-log-edit-toggle-signoff): New function.
(vc-git-log-edit-toggle-amend): New function.
(vc-git-log-edit-toggle-signoff): New function.
(vc-git-log-edit-mode): New major mode.
(vc-git-log-edit-mode-map): Keymap for it.
(vc-git-checkin): Handle "Amend" and "Sign-Off" headers.