fetch-pack: put shallow info in output parameter

fetch-pack: put shallow info in output parameter

Expand the transport fetch method signature, by adding an output
parameter, to allow transports to return information about the refs they
have fetched.  Then communicate shallow status information through this
mechanism instead of by modifying the input list of refs.

This does require clients to sometimes generate the ref map twice: once
from the list of refs provided by the remote (as is currently done) and
potentially once from the new list of refs that the fetch mechanism
provides.

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