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