test_must_fail: report number of unexpected signal

If a command is marked as test_must_fail but dies with a
signal, we consider that a problem and report the error to
stderr. However, we don't say _which_ signal; knowing that
can make debugging easier. Let's share as much as we know.

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

t/test-lib-functions.sh

@@ -617,7 +617,7 @@ test_must_fail () {
 		return 0
 	elif test $exit_code -gt 129 && test $exit_code -le 192
 	then
-		echo >&2 "test_must_fail: died by signal: $*"
+		echo >&2 "test_must_fail: died by signal $(($exit_code - 128)): $*"
 		return 1
 	elif test $exit_code -eq 127
 	then