t: translate SIGINT to an exit

Right now if a test script receives SIGINT (e.g., because a test was hanging and the user hit ^C), the shell exits immediately. This can be annoying if the test script did any global setup, like starting apache or git-daemon, as it will not have an opportunity to clean up after itself. A subsequent run of the test won't be able to start its own daemon, and will either fail or skip the tests. Instead, let's trap SIGINT to make sure we do a clean shutdown, and just chain it to a normal exit (which will trigger any cleanup). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>