fetch-pack: test cases for the new --stdin option

fetch-pack: test cases for the new --stdin option

These test cases focus only on testing the parsing of refs on stdin,
without bothering with the rest of the fetch-pack machinery. We pass in
the refs using different combinations of command line and stdin and then
we watch fetch-pack's stdout to see whether it prints all the refs we
specified (but we ignore their order).

Signed-off-by: Ivan Todoroski <grnch@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>