pfff: fix race in find_php_files
pfff: fix race in find_php_files

Summary:
Pieter discovered this bug. I also had this problem
sometimes with codegraph and never really understood why ...
until now. Thx Pieter.

Test Plan: make test

Reviewers: pieter, jiaol

Reviewed By: pieter

Differential Revision: https://phabricator.fb.com/D900661

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