Strip pkt-line when negotiating Git protocol v2

This makes it so both "version 2" and "version 2\n" (and any
alternatives with more whitespace) are accepted as a successful v2
response.

Fixes #1423

dulwich/client.py

@@ -260,7 +260,7 @@ def handle_packet(self, pkt):
 
 def negotiate_protocol_version(proto):
     pkt = proto.read_pkt_line()
-    if pkt == b"version 2\n":
+    if pkt is not None and pkt.strip() == b"version 2":
         return 2
     proto.unread_pkt_line(pkt)
     return 0