protocol: introduce enum protocol_version value protocol_v2

Introduce protocol_v2, a new value for 'enum protocol_version'.
Subsequent patches will fill in the implementation of protocol_v2.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/upload-pack.c

@@ -47,6 +47,13 @@ int cmd_upload_pack(int argc, const char **argv, const char *prefix)
 		die("'%s' does not appear to be a git repository", dir);
 
 	switch (determine_protocol_version_server()) {
+	case protocol_v2:
+		/*
+		 * fetch support for protocol v2 has not been implemented yet,
+		 * so ignore the request to use v2 and fallback to using v0.
+		 */
+		upload_pack(&opts);
+		break;
 	case protocol_v1:
 		/*
 		 * v1 is just the original protocol with a version string,