t: use test_might_fail() instead of manipulating exit code manually

These tests manually coerce the exit code of invoked commands to
"success" when they don't care if the command succeeds or fails since
failure of those commands should not cause the test to fail overall.
In doing so, they intentionally break the &&-chain. Modernize by
replacing manual exit code management with test_might_fail() and a
normal &&-chain.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t4012-diff-binary.sh b/t/t4012-diff-binary.sh
index 0a8af76..6579c81 100755
--- a/t/t4012-diff-binary.sh
+++ b/t/t4012-diff-binary.sh
@@ -102,10 +102,8 @@
 
 test_expect_success 'diff --no-index with binary creation' '
 	echo Q | q_to_nul >binary &&
-	(: hide error code from diff, which just indicates differences
-	 git diff --binary --no-index /dev/null binary >current ||
-	 true
-	) &&
+	# hide error code from diff, which just indicates differences
+	test_might_fail git diff --binary --no-index /dev/null binary >current &&
 	rm binary &&
 	git apply --binary <current &&
 	echo Q >expected &&