fast-import: insert new object entries at start of hash bucket

fast-import: insert new object entries at start of hash bucket

More often than not, find_object is called for recently inserted objects.
Optimise for this case by inserting new entries at the start of the chain.
This doesn't affect the cost of new inserts but reduces the cost of find
and insert for existing object entries.

Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>