I believe this is Twitter once again attempting to integrate their use of watchman (open sourced by FB) into the git core.

Their first attempt was in May 2014: http://www.spinics.net/lists/git/msg230487.html

Yeah, David Turner has been working for Twitter. But his work is based on previous work on an "index-helper" daemon by Nguyễn Thái Ngọc Duy who is not working for any company as far as I know.

He's also involved in the thread from May 2014.