mergetools: stop setting $status in merge_cmd()

No callers rely on $status so there's don't need to set
it during merge_cmd() for diffmerge, emerge, and kdiff3.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/diffmerge

@@ -11,5 +11,4 @@ merge_cmd () {
 		"$merge_tool_path" --merge \
 			--result="$MERGED" "$LOCAL" "$REMOTE"
 	fi
-	status=$?
 }