fetch-pack: perform a fetch using v2

When communicating with a v2 server, perform a fetch by requesting the
'fetch' command.

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

t/t5701-git-serve.sh

@@ -9,7 +9,7 @@ test_expect_success 'test capability advertisement' '
 	version 2
 	agent=git/$(git version | cut -d" " -f3)
 	ls-refs
-	fetch
+	fetch=shallow
 	0000
 	EOF