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

t/example/CC.do

@@ -2,8 +2,8 @@ redo-ifchange config.sh
 . ./config.sh
 exec >$3
 cat <<-EOF
-	redo-ifchange \$1.c
-	gcc $CFLAGS -MD -MF \$3.deps -o \$3 -c \$1.c
+	redo-ifchange \$2.c
+	gcc $CFLAGS -MD -MF \$3.deps -o \$3 -c \$2.c
 	read DEPS <\$3.deps
 	rm -f \$3.deps
 	redo-ifchange \${DEPS#*:}