t5559 breaks with apache 2.4.55

[Jeff King <peff@peff.net>]

I noticed that the test suite now fails after upgrading from apache
2.4.54 to 2.4.55 (the latter of which just hit debian unstable). The
problem is with the http2 tests, specifically t5559.30, where we send a
large fetch negotiation over http2. The output from curl is (including
some bits of tracing):

  == Info: Received 101, Switching to HTTP/2
  == Info: Using HTTP2, server supports multiplexing
  == Info: Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
  == Info: Closing connection 1
  error: RPC failed; HTTP 101 curl 16 Error in the HTTP2 framing layer

Bisecting within apache's Git repo, the culprit is their 9767274b88,
which says:

  mod_http2: version 2.0.10 of the module, synchronizing changes
  with the gitgub version. This is a partial rewrite of how connections
  and streams are handled.

which seems like a plausible source. But the diff is 8000 lines. It may
be possible to bisect within the mod_http2 source itself, but I haven't
tried it yet.

It's also not 100% clear that it's an apache bug. We could be doing
something weird with git-http-backend, or curl might be doing something
wrong. Though I tend to doubt it, given the simplicity of the CGI
interface on the server side and the fact that curl was working reliably
with older versions of apache.

So I haven't reported the bug further yet. But I thought I'd post this
here before anybody else wastes time digging in the same hole.

-Peff