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>

protocol.c

@@ -8,6 +8,8 @@ static enum protocol_version parse_protocol_version(const char *value)
 		return protocol_v0;
 	else if (!strcmp(value, "1"))
 		return protocol_v1;
+	else if (!strcmp(value, "2"))
+		return protocol_v2;
 	else
 		return protocol_unknown_version;
 }