t/t5400: demonstrate breakage caused by informational message from prune

t/t5400: demonstrate breakage caused by informational message from prune

When receive-pack triggers 'git gc --auto' and 'git prune' is called to
remove a stale temporary object, 'git prune' prints an informational
message to stdout about the file that it will remove.  Since this message
is written to stdout, it is sent back over the transport channel to the git
client which tries to interpret it as part of the pack protocol and then
promptly terminates with a complaint about a protocol error.

Introduce a test which exercises the auto-gc functionality of receive-pack
and demonstrates this breakage.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>