tests: add missing &&

Breaks in a test assertion's && chain can potentially hide
failures from earlier commands in the chain.

Commands intended to fail should be marked with !, test_must_fail, or
test_might_fail.  The examples in this patch do not require that.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t1000-read-tree-m-3way.sh b/t/t1000-read-tree-m-3way.sh
index 4f17172..ca8a409 100755
--- a/t/t1000-read-tree-m-3way.sh
+++ b/t/t1000-read-tree-m-3way.sh
@@ -309,7 +309,7 @@
 test_expect_success \
     '6 - must not exist in O && !A && !B case' "
      rm -f .git/index DD &&
-     echo DD >DD
+     echo DD >DD &&
      git update-index --add DD &&
      test_must_fail git read-tree -m $tree_O $tree_A $tree_B
 "