Revert "fetch: default to protocol version 2"

This reverts commit 684ceae32d.

Users fetching from linux-next and other kernel remotes are reporting
that the limited ref advertisement causes negotiation to reach
MAX_IN_VAIN, resulting in too-large fetches.

Reported-by: Lubomir Rintel <lkundrak@v3.sk>
Reported-by: "Dixit, Ashutosh" <ashutosh.dixit@intel.com>
Reported-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/protocol.txt

@@ -48,7 +48,7 @@ protocol.version::
 	If set, clients will attempt to communicate with a server
 	using the specified protocol version.  If the server does
 	not support it, communication falls back to version 0.
-	If unset, the default is `2`.
+	If unset, the default is `0`.
 	Supported versions:
 +
 --

protocol.c

@@ -39,7 +39,7 @@ enum protocol_version get_protocol_version_config(void)
 		return env;
 	}
 
-	return protocol_v2;
+	return protocol_v0;
 }
 
 enum protocol_version determine_protocol_version_server(void)