Don't create directories as 0777 by default.

Summary: Currently, Filesystem::createDirectory('...') will create a directory with (rather open) 0777 permissions (regardless of the return value of umask()). In particular, the .git/arc directory (or similar directory for another VCS) will have 0777 permissions.

Test Plan: Removed the .git/arc directory and ran arc lint. Verified that the .git/arc directory had more restrictive permissions than 0777.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9594