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.

This patch support force enable HTTP/2 or HTTP/1.1.

example:

GIT_CURL_VERBOSE=1 git2 -c http.version=HTTP/2 ls-remote https://bitbucket.org/aquariusjay/deeplab-public-ver2.git

Force Charlie (1):
http: add support selecting http version

 Documentation/config.txt |  9 +++++++++
 http.c                   | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

base-commit: 8858448bb49332d353febc078ce4a3abcc962efe
Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-69%2Ffcharlie%2Fmaster-v8
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-69/fcharlie/master-v8
Pull-Request: https://github.com/gitgitgadget/git/pull/69

Range-diff vs v7:

1: e26fc0d8c7 ! 1: 71f8b71b34 http: add support selecting http version

     @@ -78,6 +78,7 @@
      +		}
      +	}
      +
     ++	warning("unknown value given to http.version: '%s'", version_string);
      +	return -1; /* not found */
      +}
      +

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