pack-objects: declare 'rev_info' for '--stdin-packs' earlier

pack-objects: declare 'rev_info' for '--stdin-packs' earlier

Once 'read_packs_list_from_stdin()' has called for_each_object_in_pack()
on each of the input packs, we do a reachability traversal to discover
names for any objects we picked up so we can generate name hash values
and hopefully get higher quality deltas as a result.

A future commit will change the purpose of this reachability traversal
to find and pack objects which are reachable from commits in the input
packs, but are packed in an unknown (not included nor excluded) pack.

Extract the code which initializes and performs the reachability
traversal to take place in the caller, not the callee, which prepares us
to share this code for the '--unpacked' case (see the function
add_unreachable_loose_objects() for more details).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>