Font lock long Git commit summary lines

Font lock long Git commit summary lines

* lisp/vc/vc-git.el (vc-git-log-edit-summary-target-len)
(vc-git-log-edit-summary-max-len): New defcustoms.
(vc-git-log-edit-summary-target-warning)
(vc-git-log-edit-summary-max-warning): New faces.
(vc-git--log-edit-summary-check): New function.
(vc-git-log-edit-mode): Add vc-git--log-edit-summary-check to
log-edit-font-lock-keywords to font lock long Git commit summary
lines.
* etc/NEWS (VC): Document the change.
* .dir-locals.el: Set vc-git-log-edit-summary-target-len.