(smerge-mode-menu): Fix activate pred for resolve.

(smerge-mode-menu): Fix activate pred for resolve.
(smerge-context-menu-map): Remove unused var.
(smerge-keep-all): Preserve markers.
(smerge-keep-n): New fun.
(smerge-keep-base, smerge-keep-other, smerge-keep-mine)
(smerge-keep-current, smerge-ediff): Use it.
(smerge-kill-current): Use it.  Make it work on some 3-part conflicts.
(smerge-popup-context-menu): Also use context-menu on 3-part conflicts.
(smerge-resolve): Resolve trivial 3-part conflicts.