allow specifying a mount for 'eden debug gc_process_fetch'
allow specifying a mount for 'eden debug gc_process_fetch'

Summary: If the user specifies a mount, only clear for that mount, otherwise clear for all mounts.

Reviewed By: fanzeyi

Differential Revision: D22674210

fbshipit-source-id: 832c4fd37a63fdb44cb12844378b61d113731016

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