If using --log or -j > 1, disable stdin.

Parallelism and redo-log cause lots of confusion for any rules that try
to ask the user for questions, so disable it altogether.

Arguably, we should just disable stdin all the time, but maybe it's
still occasionally useful (even though you have to pass --no-log to get
it back).

redo-ifchange.py

@@ -19,7 +19,8 @@ def should_build(t):
 
 rv = 202
 try:
-    if vars_init.is_toplevel:
+    if vars_init.is_toplevel and vars.LOG:
+        builder.close_stdin()
         builder.start_stdin_log_reader(status=True, details=True,
             pretty=True, color=True, debug_locks=False, debug_pids=False)
     if vars.TARGET and not vars.UNLOCKED: