Watchman: spawn watchman server

Summary:
Since watchman 2.9.8, we need to provide an input command
get-pid could be the default one, watchman will be spawn.

Test Plan: Upgrade watchman to 2.9.8 and run this diff

Reviewers: adrienconrath

Reviewed By: adrienconrath

Subscribers: NicolasDP

Differential Revision: http://91.121.199.76/D64