t5516: beef up exact-oid ref prefixes test

Commit 6c301adb0a (fetch: do not pass ref-prefixes for fetch by exact SHA1, 2018-05-31) added a test that fetching an exact oid with the v2 protocol works. Originally it failed without the code change from that commit, because fetch failed with "no matching remote head". That changed in 0177565148 (transport: do not list refs if possible, 2018-09-27), which made fetch more forgiving of this case. But that now meant the test passes even without its fix! So let's also have it check the packet listing to make sure we did not ask for the bogus prefix (ultimately this is less important than whether the command fails, since it's just an optimization, but we should make sure not to regress it). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>