Our documentation claims we support curl versions back to 7.19.5. But we
can no longer compile with that version since adding an unconditional
use of CURLOPT_RESOLVE in 511cfd3 (http: add custom hostname to IP
address resolutions, 2022-05-16). That feature wasn't added to libcurl
until 7.21.3.

We could add #ifdefs to make this work back to 7.19.5. But given that
nobody noticed the compilation failure in the intervening two years, it
makes more sense to bump the version in the documentation to 7.21.3
(which is itself over 13 years old).

We could perhaps go forward even more (which would let us drop some
cruft from git-curl-compat.h), but this should be an obviously safe
jump, and we can move forward later.

Note that user-visible syntax for CURLOPT_RESOLVE has grown new features
in subsequent curl versions. Our documentation mentions "+" and "-"
entries, which require more recent versions than 7.21.3. We could
perhaps clarify that in our docs, but it's probably not worth cluttering
them with restrictions of ancient curl versions.

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

As of curl 7.66.0, we don't need to manually specify a "chunked"
Transfer-Encoding header. Instead, modern curl deduces the need for it
in a POST that has a POSTFIELDSIZE of -1 and uses READFUNCTION rather
than POSTFIELDS.

That version is recent enough that we can't just drop the header; we
need to do so conditionally. Since it's only a single line, it seems
like the simplest thing would just be to keep setting it unconditionally
(after all, the #ifdefs are much longer than the actual code). But
there's another wrinkle: HTTP/2.

Curl may choose to use HTTP/2 under the hood if the server supports it.
And in that protocol, we do not use the chunked encoding for streaming
at all. Most versions of curl handle this just fine by recognizing and
removing the header. But there's a regression in curl 8.7.0 and 8.7.1
where it doesn't, and large requests over HTTP/2 are broken (which t5559
notices). That regression has since been fixed upstream, but not yet
released.

Make the setting of this header conditional, which will let Git work
even with those buggy curl versions. And as a bonus, it serves as a
reminder that we can eventually clean up the code as we bump the
supported curl versions.

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