don't recrawl if a stale watch descriptor is found

Summary: I believe this is the correct fix.

Test Plan:
`make integration`, and hgwatchman's test suite -- made sure we never recrawled. Also ran `hg update .~10000` in a large repo -- we'd just about always recrawl without this change. With this change we never recrawled.

Finally, as a sanity check, compared the output of `find -type f` and a watchman query for all files right after the update. Saw that except files in `.hg` there were no discrepancies.

Reviewers: wez

Reviewed By: wez

Subscribers: dreiss

Differential Revision: https://phabricator.fb.com/D1488238