t/t4*: merge a "grep | sed" pipeline

Signed-off-by: Beat Bolli <[email protected]>
Acked-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Beat Bolli

t/t4002-diff-basic.sh

@@ -405,7 +405,7 @@ test_expect_success 'diff-tree -r B A == diff-tree -r -R A B' '
 
 test_expect_success 'diff can read from stdin' '
 	test_must_fail git diff --no-index -- MN - < NN |
-		grep -v "^index" | sed "s#/-#/NN#" >.test-a &&
+		sed "/^index/d; s#/-#/NN#" >.test-a &&
 	test_must_fail git diff --no-index -- MN NN |
 		grep -v "^index" >.test-b &&
 	test_cmp .test-a .test-b