t9813: avoid using pipes

The exit code of the upstream in a pipe is ignored thus we should avoid
using it. By writing out the output of the git command to a file, we can
test the exit codes of both the commands.

Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Acked-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9813-git-p4-preserve-users.sh

@@ -118,12 +118,12 @@ test_expect_success 'not preserving user with mixed authorship' '
 		make_change_by_user usernamefile3 Derek derek@example.com &&
 		P4EDITOR=cat P4USER=alice P4PASSWD=secret &&
 		export P4EDITOR P4USER P4PASSWD &&
-		git p4 commit |\
-		grep "git author derek@example.com does not match" &&
+		git p4 commit >actual &&
+		grep "git author derek@example.com does not match" actual &&
 
 		make_change_by_user usernamefile3 Charlie charlie@example.com &&
-		git p4 commit |\
-		grep "git author charlie@example.com does not match" &&
+		git p4 commit >actual &&
+		grep "git author charlie@example.com does not match" actual &&
 
 		make_change_by_user usernamefile3 alice alice@example.com &&
 		git p4 commit >actual &&