Add special markup processing for commit logs.

Add special markup processing for commit logs.
* log-edit.el (log-edit-extra-flags): New variable.
(log-edit): Add new argument MODE.  Use that mode when non-nil
instead of the log-view-mode.
(log-view-process-buffer): New function.

* vc.el: Document that the checkin method takes optional
arguments.  Document new backend specific method: log-view-mode.
(vc-default-log-edit-mode): New function.
(vc-checkin): Use a backend specific log-view-mode.
Pass extra arguments to the checkin method.
(vc-modify-change-comment): Pass a dummy extra argument.

* vc-dispatcher.el (vc-log-edit): Add a mode argument, pass it to
log-edit.
(vc-start-logentry): Add a mode argument, pass it to vc-log-edit.
(vc-finish-logentry): Process the log buffer before passing it
down.  Pass log-edit-extra-flags.

* vc-bzr.el (vc-bzr-checkin): Pass extra arguments to the commit
command.
(log-edit-extra-flags, log-edit-before-checkin-process): New declarations.

* vc-hg.el (vc-hg-checkin): Pass extra arguments to the commit
command.
(log-edit-extra-flags, log-edit-before-checkin-process): New declarations.
(vc-hg-log-edit-mode): New derived mode.

* vc-arch.el (vc-arch-checkin):
* vc-cvs.el (vc-cvs-checkin):
* vc-git.el (vc-git-checkin):
* vc-mtn.el (vc-mtn-checkin):
* vc-rcs.el (vc-rcs-checkin):
* vc-sccs.el (vc-sccs-checkin):
* vc-svn.el (vc-svn-checkin): Add an optional ignored argument.