Update `.gitignore`.
Update `.gitignore`.

Summary: See inline comments for reasoning.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9530

julkiewicz/libphutil,julkiewicz/libphutil,julkiewicz/libphutil