redo: only default to 'all' in the toplevel instance of redo.

We already did this in minimal/do, and redo-ifchange already did this, but
plain redo didn't.  This made constructs like:

	for d in *.x; do
		echo "${d%.x}"
	done | xargs redo

dangerous, because if there were no matching files, we'd try to 'redo all'.

vars_init.py

@@ -3,6 +3,8 @@ import sys, os
 def init(targets):
     if not os.environ.get('REDO'):
         # toplevel call to redo
+        if len(targets) == 0:
+            targets.append('all')
         exenames = [os.path.abspath(sys.argv[0]),
                     os.path.realpath(sys.argv[0])]
         dirnames = [os.path.dirname(p) for p in exenames]