Merge branch 'lv/tls-1.3'

When built with more recent cURL, GIT_SSL_VERSION can now specify
"tlsv1.3" as its value.

* lv/tls-1.3:
  http: allow use of TLS 1.3

Documentation/config.txt

@@ -1957,6 +1957,7 @@ http.sslVersion::
 	- tlsv1.0
 	- tlsv1.1
 	- tlsv1.2
+	- tlsv1.3
 
 +
 Can be overridden by the `GIT_SSL_VERSION` environment variable.

http.c

@@ -63,6 +63,9 @@ 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;