improve a debug logging message
improve a debug logging message

Summary:
The pending event flags transition during crawl is important
information to see when debugging incorrect query results.

Reviewed By: genevievehelsel

Differential Revision: D29412377

fbshipit-source-id: 39e615858ee4e5985fed9a8cb72d4710fb69348a

nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman