pack-objects: better check_object() performances

With large amount of objects, check_object() is really trashing the pack
sliding map and the filesystem cache.  It has a completely random access
pattern especially with old objects where delta replay jumps back and
forth all over the pack.

This patch improves things by:

 1) sorting objects by their offset in pack before calling check_object()
    so the pack access pattern is linear;

 2) recording the object type at add_object_entry() time since it is
    already known in most cases;

 3) recording the pack offset even for preferred_base objects;

 4) avoid calling sha1_object_info() if all possible.

This limits pack accesses to the bare minimum and makes them perfectly
linear.

In the process check_object() was made more clear (to me at least).

Note: I thought about walking the sorted_by_offset list backward in
get_object_details() so if a pack happens to be larger than the available
file cache, then the cache would have been populated with useful data from
the beginning of the pack already when find_deltas() is called.  Strangely,
testing (on Linux) showed absolutely no performance difference.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

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