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/defaults-flat/default.b.do

@@ -1,10 +1,10 @@
-if [ -e "$1$2.a" -o -e "default$2.a" ]; then
-	redo-ifchange "$1$2.a"
+if [ -e "$1.a" -o -e "default${1#$2}.a" ]; then
+	redo-ifchange "$1.a"
 	echo a-to-b
-	cat "$1$2.a"
+	cat "$1.a"
 else
-	redo-ifchange "$1$2.b"
+	redo-ifchange "$1.b"
 	echo b-to-b
-	cat "$1$2.b"
+	cat "$1.b"
 fi
 ../sleep 1.1