Change definitions of $1,$2,$3 to match djb's redo.

If you use "redo --old-args", it will switch back to the old
(apenwarr-style) arguments for now, to give you time to update your .do
scripts.  This option will go away eventually.

Note: minimal/do doesn't understand the --old-args option.  If you're using
minimal/do in your project, keep using the old one until you update your use
of $1/$2, and then update to the new one.

apenwarr-style default.o.do:
   $1      foo
   $2      .o
   $3      whatever.tmp

djb-style default.o.do:
   $1      foo.o
   $2      foo
   $3      whatever.tmp

apenwarr-style foo.o.do:
   $1      foo.o
   $2      ""
   $3      whatever.tmp

djb-style foo.o.do:
   $1      foo.o
   $2      foo.o  (I think?)
   $3      whatever.tmp

minimal/do

@@ -121,7 +121,7 @@ _do()
 		fi
 		[ ! -e "$DO_BUILT" ] || [ ! -d "$(dirname "$target")" ] ||
 		: >>"$target.did"
-		( _run_dofile "$base" "$ext" "$tmp.tmp" )
+		( _run_dofile "$target" "$base" "$tmp.tmp" )
 		rv=$?
 		if [ $rv != 0 ]; then
 			printf "do: %s%s\n" "$DO_DEPTH" \