Simplify diff exit code handling.
Simplify diff exit code handling.

Summary:
This fixes T9970 in an alternate manner, with the same effect: the
binary_safe_diff.sh script returns 0 if the diff succeeds, 1 in all
other cases.

Test Plan:
Tested locally with a fixed binary_safe_diff.sh, resulting in this
correct review:

https://reviews.freebsd.org/D4542

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: eadler, Korvin, stevenh

Maniphest Tasks: T9970

Differential Revision: https://secure.phabricator.com/D14759

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