t6101: avoid suppressing git's exit code

Update t6101-rev-parse-parents.sh to redirect git-rev-parse
output to a temporary file instead of piping it directly to
not hide the exit code of git commands behind pipes, as a
crash in git might go unnoticed.

Signed-off-by: Trieu Huynh <vikingtc4@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: trieu1162000

t/t6101-rev-parse-parents.sh

@@ -39,7 +39,8 @@ test_expect_success 'setup' '
 '
 
 test_expect_success 'start is valid' '
-	git rev-parse start | grep "^$OID_REGEX$"
+	git rev-parse start >actual &&
+	test_grep "^$OID_REGEX$" actual
 '
 
 test_expect_success 'start^0' '