'make clean' can now safely remove .redo dirs.

.gitignore

@@ -1,3 +1,9 @@
 *~
 *.pyc
 *.tmp
+.redo
+t/CC
+t/LD
+t/[yb]ellow
+t/hello
+t/*.o

clean.do

@@ -1 +1,2 @@
 rm -f t/hello t/[by]ellow t/*.o *~ .*~ t/*~ t/.*~ *.pyc t/CC t/LD
+rm -rf .redo

redo.py

@@ -103,6 +103,9 @@ def dirty_deps(t, depth):
 
 def stamp(t):
     stampfile = sname('stamp', t)
+    if not os.path.exists(REDO_BASE + '/.redo'):
+        # .redo might not exist in a 'make clean' target
+        return
     open(stampfile, 'w').close()
     try:
         mtime = os.stat(t).st_mtime