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