fetch-pack: add specific error for fetching an unadvertised object

fetch-pack: add specific error for fetching an unadvertised object

Enhance filter_refs (which decides whether a request for an unadvertised
object should be sent to the server) to record a new match status on the
"struct ref" when a request is not allowed, and have
report_unmatched_refs check for this status and print a special error
message, "Server does not allow request for unadvertised object".

Signed-off-by: Matt McCutchen <redacted>
Signed-off-by: Junio C Hamano <redacted>