Minor whitespace changes.

Summary:
A few minor whitespace changes:

- Change tabs to spaces.
- Don't change tabs to spaces in test cases.
- Don't apply `.editorconfig` rules to the `externals` directory.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9673