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>

Documentation/config.txt

@@ -1935,6 +1935,15 @@ http.saveCookies::
 	If set, store cookies received during requests to the file specified by
 	http.cookieFile. Has no effect if http.cookieFile is unset.
 
+http.version::
+	Use the specified HTTP protocol version when communicating with a server.
+	If you want to force the default. The available and default version depend
+	on libcurl. Actually the possible values of
+	this option are:
+
+	- HTTP/2
+	- HTTP/1.1
+
 http.sslVersion::
 	The SSL version to use when negotiating an SSL connection, if you
 	want to force the default.  The available and default version

http.c

@@ -48,6 +48,7 @@ char curl_errorstr[CURL_ERROR_SIZE];
 
 static int curl_ssl_verify = -1;
 static int curl_ssl_try;
+static const char *curl_http_version = NULL;
 static const char *ssl_cert;
 static const char *ssl_cipherlist;
 static const char *ssl_version;
@@ -284,6 +285,9 @@ static void process_curl_messages(void)
 
 static int http_options(const char *var, const char *value, void *cb)
 {
+	if (!strcmp("http.version", var)) {
+		return git_config_string(&curl_http_version, var, value);
+	}
 	if (!strcmp("http.sslverify", var)) {
 		curl_ssl_verify = git_config_bool(var, value);
 		return 0;
@@ -789,6 +793,31 @@ static long get_curl_allowed_protocols(int from_user)
 }
 #endif
 
+#if LIBCURL_VERSION_NUM >=0x072f00
+static int get_curl_http_version_opt(const char *version_string, long *opt)
+{
+	int i;
+	static struct {
+		const char *name;
+		long opt_token;
+	} choice[] = {
+		{ "HTTP/1.1", CURL_HTTP_VERSION_1_1 },
+		{ "HTTP/2", CURL_HTTP_VERSION_2 }
+	};
+
+	for (i = 0; i < ARRAY_SIZE(choice); i++) {
+		if (!strcmp(version_string, choice[i].name)) {
+			*opt = choice[i].opt_token;
+			return 0;
+		}
+	}
+
+	warning("unknown value given to http.version: '%s'", version_string);
+	return -1; /* not found */
+}
+
+#endif
+
 static CURL *get_curl_handle(void)
 {
 	CURL *result = curl_easy_init();
@@ -806,6 +835,16 @@ static CURL *get_curl_handle(void)
 		curl_easy_setopt(result, CURLOPT_SSL_VERIFYHOST, 2);
 	}
 
+#if LIBCURL_VERSION_NUM >= 0x072f00 // 7.47.0
+    if (curl_http_version) {
+		long opt;
+		if (!get_curl_http_version_opt(curl_http_version, &opt)) {
+			/* Set request use http version */
+			curl_easy_setopt(result, CURLOPT_HTTP_VERSION, opt);
+		}
+    }
+#endif
+
 #if LIBCURL_VERSION_NUM >= 0x070907
 	curl_easy_setopt(result, CURLOPT_NETRC, CURL_NETRC_OPTIONAL);
 #endif