(smerge-basic-map): Bind smerge-combine-with-next.

(smerge-basic-map): Bind smerge-combine-with-next.
(smerge-auto-leave): Make undo re-enable the mode.
(debug-ignored-errors): Add the user-error of impossible resolution.
(smerge-resolve): Move things around a bit, in preparation for later.
(smerge-diff): Don't fail if the buffer has no associated file.
(ediff-ancestor-buffer, ediff-quit-hook): Quieten byte-compiler.
(smerge-conflict-overlay): New function.
(smerge-match-conflict): Don't add text properties here.
(smerge-find-conflict): Instead, add an overlay here.
Also check for the case where we're inside a conflict already, so as to
obviate the need for font-lock-multiline, which is unbearably slow with
large conflicts and ciomplex font-lock patterns.
(smerge-remove-props): Remove overlay rather than text-properties.
(smerge-mode): Don't set font-lock-multiline.
Remove overlays when turned off.