t3510 (cherry-pick-sequence): add missing '&&'

Breaks in a test assertion's && chain can potentially hide failures
from earlier commands in the chain.  Fix an instance of this in the
setup.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3510-cherry-pick-sequence.sh

@@ -28,7 +28,7 @@ test_cmp_rev () {
 }
 
 test_expect_success setup '
-	git config advice.detachedhead false
+	git config advice.detachedhead false &&
 	echo unrelated >unrelated &&
 	git add unrelated &&
 	test_commit initial foo a &&