Change -x/-v to only affect top-level targets by default, not recursively.

Because redo targets are nicely isolated (unlike make targets), you
usually only want to debug one of them at a time.  Using -x could be
confusing, because you might end up with a dump of output from a
dependency you're not interested in.

Now, by default we'll disable -x when recursing into sub-targets, so
you only see the trace from the targets you are actually trying to
debug.  To get recursive behaviour, specify -x twice, eg. -xx.

Same idea with -v.

redo/cmd_redo.py

@@ -53,9 +53,9 @@ def main():
     if opt.debug:
         os.environ['REDO_DEBUG'] = str(opt.debug or 0)
     if opt.verbose:
-        os.environ['REDO_VERBOSE'] = '1'
+        os.environ['REDO_VERBOSE'] = str(opt.verbose)
     if opt.xtrace:
-        os.environ['REDO_XTRACE'] = '1'
+        os.environ['REDO_XTRACE'] = str(opt.xtrace)
     if opt.keep_going:
         os.environ['REDO_KEEP_GOING'] = '1'
     if opt.shuffle: