mergetool--lib: specialize diff options for emerge and ecmerge

The ecmerge documentation mentions the following form:

	ecmerge --mode=diff2 $1 $2

Since git-difftool is about diffing, we should use that instead
of --mode=merge2.  Likewise, this drops the $MERGED argument to
emerge, as discussed on the git list ($gmane/117930).

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

git-mergetool--lib.sh

@@ -228,8 +228,8 @@ run_merge_tool () {
 			fi
 			check_unchanged
 		else
-			"$merge_tool_path" "$LOCAL" "$REMOTE" \
-				--default --mode=merge2 --to="$MERGED"
+			"$merge_tool_path" --default --mode=diff2 \
+				"$LOCAL" "$REMOTE"
 		fi
 		;;
 	emerge)
@@ -248,7 +248,7 @@ run_merge_tool () {
 			status=$?
 		else
 			"$merge_tool_path" -f emerge-files-command \
-				"$LOCAL" "$REMOTE" "$(basename "$MERGED")"
+				"$LOCAL" "$REMOTE"
 		fi
 		;;
 	tortoisemerge)