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

phacility/arcanist,folsom-labs/arcanist,Khan/arcanist,fivestars/arcanist,fivestars/arcanist,jra3/arcanist,memsql/arcanist,coursera/arcanist,zhihu/arcanist,jra3/arcanist,wikimedia/phabricator-arcanist,jra3/arcanist,r4nt/arcanist,haskell-infra/arcanist,fivestars/arcanist,codelabs-ch/arcanist,chundongwang/arcanist,calblueprint/arcanist,codelabs-ch/arcanist,Khan/arcanist,uber/arcanist,uber/arcanist,memsql/arcanist,chundongwang/arcanist,memsql/arcanist,phacility/arcanist,phacility/arcanist,codelabs-ch/arcanist,coursera/arcanist,phacility/arcanist,haskell-infra/arcanist,coursera/arcanist,chundongwang/arcanist,codelabs-ch/arcanist,r4nt/arcanist,r4nt/arcanist,coursera/arcanist,Khan/arcanist,folsom-labs/arcanist,haskell-infra/arcanist,r4nt/arcanist,aik099/arcanist,calblueprint/arcanist,uber/arcanist,Khan/arcanist,zhihu/arcanist,uber/arcanist,wikimedia/phabricator-arcanist,aik099/arcanist,jra3/arcanist,codelabs-ch/arcanist,folsom-labs/arcanist,wikimedia/phabricator-arcanist,zhihu/arcanist,phacility/arcanist,wikimedia/phabricator-arcanist,folsom-labs/arcanist,calblueprint/arcanist,aik099/arcanist,calblueprint/arcanist,fivestars/arcanist,haskell-infra/arcanist,zhihu/arcanist,chundongwang/arcanist,memsql/arcanist,aik099/arcanist