Added context menu support in smerge mode.
Most of the part is written by Stefan Monnier.
* smerge-mode.el (smerge-context-menu-map, smerge-context-menu): New
keyman and menu.
(smerge-text-properties): New function.
(smerge-remove-props): New function.
(smerge-popup-context-menu): New function.
(smerge-resolve): Call `smerge-remove-props'.
(smerge-keep-base, smerge-keep-other, smerge-keep-mine):
Ditto.
(smerge-keep-current): Ditto.
(smerge-kill-current): New function.
(smerge-match-conflict): Detect the file as `a same-diff conflict'
if the filename is "ANCESTOR". Put text properties.