t5551: test usage of chunked encoding explicitly

t5551: test usage of chunked encoding explicitly

When run using GIT_TEST_PROTOCOL_VERSION=2, a test in t5551 fails
because 4 POSTs (probe, ls-refs, probe, fetch) are sent instead of 2
(probe, fetch).

One way to resolve this would be to relax the condition (from "= 2" to
greater than 1, say), but upon further inspection, the test probably
shouldn't be counting the number of POSTs. This test states that large
requests are split across POSTs, but this is not correct; the main
change is that chunked transfer encoding is used, but the request is
still contained within one POST. (The test coincidentally works because
Git indeed sends 2 POSTs in the case of a large request, but that is
because, as stated above, the first POST is a probing RPC - see
post_rpc() in remote-curl.c for more information.)

Therefore, instead of counting POSTs, check that chunked transfer
encoding is used. This also has the desirable side effect of passing
with GIT_TEST_PROTOCOL_VERSION=2.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>