t5551: test v2-to-v0 http protocol fallback

Since we use the v2 protocol by default, the connection of a v2 client to a v2 server is well covered by the test suite. And with the GIT_TEST_PROTOCOL_VERSION knob, we can easily test a v0 client connecting to a v2-aware server (which will then just speak v0). But we have no regular tests that a v2 client, when encountering a non-v2-aware server, will correctly fall back to using v0. In theory this is a job for the cross-version tests in t/interop, but: - they cover only git:// and file:// clones - they are not part of the usual test suite, so nobody ever runs them anyway Since using v2 over http requires configuring the web server to pass along the Git-Protocol header, we can easily create a situation where the server does not respect the v2 probe, and the conversation falls back to v0. This works just fine. This new test is not about fixing any particular bug, but just making sure that the system works (and continues to work) as expected. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>