filemerge: use 'util.ellipsis' to trim custom conflict markers correctly Before this patch, filemerge slices byte sequence directly to trim conflict markers, but this may cause: - splitting at intermediate multi-byte sequence - incorrect calculation of column width (length of byte sequence is different from columns in display in many cases) This patch uses 'util.ellipsis' to trim custom conflict markers correctly, even if multi-byte characters are used in them.