t5520: replace ! git with test_must_fail git

Currently, if a git command fails in an unexpected way, such as a
segfault, it will be masked and ignored. Replace the ! with
test_must_fail so that only expected failures pass.

Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

t/t5520-pull.sh

@@ -537,7 +537,7 @@ test_expect_success 'pull --rebase=i' '
 test_expect_success 'pull.rebase=invalid fails' '
 	git reset --hard before-preserve-rebase &&
 	test_config pull.rebase invalid &&
-	! git pull . copy
+	test_must_fail git pull . copy
 '
 
 test_expect_success '--rebase=false create a new merge commit' '
@@ -572,7 +572,7 @@ test_expect_success REBASE_P \
 
 test_expect_success '--rebase=invalid fails' '
 	git reset --hard before-preserve-rebase &&
-	! git pull --rebase=invalid . copy
+	test_must_fail git pull --rebase=invalid . copy
 '
 
 test_expect_success '--rebase overrides pull.rebase=preserve and flattens keep-merge' '