tests: make test_might_fail more verbose

Let test_might_fail say something about its failures for consistency
with test_must_fail.

Cc: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jrn

t/test-lib.sh

@@ -618,7 +618,12 @@ test_must_fail () {
 
 test_might_fail () {
 	"$@"
-	test $? -ge 0 -a $? -le 129 -o $? -gt 192
+	exit_code=$?
+	if test $exit_code -gt 129 -a $exit_code -le 192; then
+		echo >&2 "test_might_fail: died by signal: $*"
+		return 1
+	fi
+	return 0
 }
 
 # test_cmp is a helper function to compare actual and expected output.