commit e7b7bf65b489c5890c4f64245769b8dc5c7fd916
Author:     Stefan Monnier <monnier@iro.umontreal.ca>
AuthorDate: Wed Aug 29 11:11:51 2012 -0400

    * lisp/files.el (read-only-mode): New minor mode.
    (toggle-read-only): Use it and mark obsolete.
    (find-file--read-only):
    * lisp/vc/vc.el (vc-next-action, vc-checkout):
    * lisp/vc/vc-cvs.el (vc-cvs-checkout):
    * lisp/obsolete/vc-mcvs.el (vc-mcvs-update):
    * lisp/ffap.el (ffap--toggle-read-only): Update callers.