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

builder.py

@@ -141,10 +141,16 @@ class BuildJob:
         close_on_exec(ffd, True)
         self.f = os.fdopen(ffd, 'w+')
         # this will run in the dofile's directory, so use only basenames here
+        if vars.OLD_ARGS:
+            arg1 = basename  # target name (no extension)
+            arg2 = ext       # extension (if any), including leading dot
+        else:
+            arg1 = basename + ext  # target name (including extension)
+            arg2 = basename        # target name (without extension)
         argv = ['sh', '-e',
                 dofile,
-                basename, # target name (no extension)
-                ext,  # extension (if any), including leading dot
+                arg1,
+                arg2,
                 # temp output file name
                 state.relpath(os.path.abspath(self.tmpname2), dodir),
                 ]