Preserve the exit code of the diff command in binary_safe_diff.sh

Summary:
Some versions of Subversion (1.9 in any case, maybe others) will
duplicate diff headers, if the diff command run through --diff-cmd
returns 0.

This lead to T9970, where the addition of a new file with properties
only shows the properties themselves in the review, not the content of
the new file.

Test Plan: This is a trivial change, is a test needed at all?

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: stevenh, Korvin, eadler

Differential Revision: https://secure.phabricator.com/D14755