remote-curl: let users turn off smart http

remote-curl: let users turn off smart http

Usually there is no need for users to specify whether an
http remote is smart or dumb; the protocol is designed so
that a single initial request is made, and the client can
determine the server's capability from the response.

However, some misconfigured dumb-only servers may not like
the initial request by a smart client, as it contains a
query string. Until recently, commit 703e6e7 worked around
this by making a second request. However, that commit was
recently reverted due to its side effect of masking the
initial request's error code.

Since git has had that workaround for several years, we
don't know exactly how many such misconfigured servers are
out there. The reversion of 703e6e7 assumes they are rare
enough not to worry about. Still, that reversion leaves
somebody who does run into such a server with no escape
hatch at all. Let's give them an environment variable they
can tweak to perform the "dumb" request.

This is intentionally not a documented interface. It's
overly simple and is really there for debugging in case
somebody does complain about git not working with their
server. A real user-facing interface would entail a
per-remote or per-URL config variable.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>