http: allow use of TLS 1.3

Add a tlsv1.3 option to http.sslVersion in addition to the existing
tlsv1.[012] options. libcurl has supported this since 7.52.0.

This requires OpenSSL 1.1.1 with TLS 1.3 enabled or curl built with
recent versions of NSS or BoringSSL as the TLS backend.

Signed-off-by: Loganaden Velvindron <logan@hackers.mu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1948,6 +1948,7 @@ http.sslVersion::
 	- tlsv1.0
 	- tlsv1.1
 	- tlsv1.2
+	- tlsv1.3
 
 +
 Can be overridden by the `GIT_SSL_VERSION` environment variable.

http.c

@@ -62,6 +62,9 @@ static struct {
 	{ "tlsv1.1", CURL_SSLVERSION_TLSv1_1 },
 	{ "tlsv1.2", CURL_SSLVERSION_TLSv1_2 },
 #endif
+#if LIBCURL_VERSION_NUM >= 0x073400
+	{ "tlsv1.3", CURL_SSLVERSION_TLSv1_3 },
+#endif
 };
 #if LIBCURL_VERSION_NUM >= 0x070903
 static const char *ssl_key;