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'.

redo.py

@@ -19,7 +19,7 @@ version    print the current version and exit
 o = options.Options('redo', optspec)
 (opt, flags, extra) = o.parse(sys.argv[1:])
 
-targets = extra or ['all']
+targets = extra
 
 if opt.version:
     import version

t/blank/blank.do

@@ -1,2 +1,3 @@
 redo-ifchange
-
+redo-ifcreate
+redo

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]