t9001: enhance fake sendmail test harness
t9001: enhance fake sendmail test harness

Previously, the fake.sendmail test harness would write its
output to a hardcoded file, allowing only a single message
to be tested. Instead, let's have it save the messages for
all of its invocations so that we can see which messages
were sent, and in which order.

Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git