http: add support selecting http version

http: add support selecting http version

Usually we don't need to set libcurl to choose which version of the
HTTP protocol to use to communicate with a server.
But different versions of libcurl, the default value is not the same.

CURL >= 7.62.0: CURL_HTTP_VERSION_2TLS
CURL < 7.62: CURL_HTTP_VERSION_1_1

In order to give users the freedom to control the HTTP version,
we need to add a setting to choose which HTTP version to use.

Signed-off-by: Force Charlie <charlieio@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>