upload-pack: introduce fetch server command

Introduce the 'fetch' server command.

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

serve.c

@@ -6,6 +6,7 @@
 #include "argv-array.h"
 #include "ls-refs.h"
 #include "serve.h"
+#include "upload-pack.h"
 
 static int always_advertise(struct repository *r,
 			    struct strbuf *value)
@@ -54,6 +55,7 @@ struct protocol_capability {
 static struct protocol_capability capabilities[] = {
 	{ "agent", agent_advertise, NULL },
 	{ "ls-refs", always_advertise, ls_refs },
+	{ "fetch", always_advertise, upload_pack_v2 },
 };
 
 static void advertise_capabilities(void)