* lisp/vc/smerge-mode.el (smerge-refine-regions): Work in multi-bufs

* lisp/vc/smerge-mode.el (smerge-refine-regions): Work in multi-bufs

Rename from smerge-refine-subst.  Allow the `beg's to be markers.
Add autoload cookie.
(smerge--refine-forward): Rename from smerge-refine-forward.
(smerge--refine-chopup-region): Rename from smerge-refine-chopup-region.
Assume that its `beg` arg is a marker.
(smerge--refine-highlight-change): Rename from
smerge-refine-highlight-change.  Remove `buf` arg.
(smerge-refine-subst): Redefine as an obsolete alias.