test_must_fail: use test_match_signal

In 8bf4bec (add "ok=sigpipe" to test_must_fail and use it to
fix flaky tests, 2015-11-27), test_must_fail learned to
recognize "141" as a sigpipe failure. However, testing for
a signal is more complicated than that; we should use
test_match_signal to implement more portable checking.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -612,7 +612,7 @@ test_must_fail () {
 	then
 		echo >&2 "test_must_fail: command succeeded: $*"
 		return 1
-	elif test $exit_code -eq 141 && list_contains "$_test_ok" sigpipe
+	elif test_match_signal 13 $exit_code && list_contains "$_test_ok" sigpipe
 	then
 		return 0
 	elif test $exit_code -gt 129 && test $exit_code -le 192