patch, 2 partsAvoid spending cycles to deltify binary guck

Somebody wondered on #git if there is a way to avoid spending cycles and memory on certain objects in a repository, most notably huge image files (or it might have been porn video collection, but I do not recall the details).

Here is a quick-and-clean patch series to help such a repository.

 [PATCH 1/2] pack-objects: pass fullname down to add_object_entry()
 [PATCH 2/2] Teach "delta" attribute to pack-objects.

Although we give pack-objects the pathname for each blob, we used to hash it down before registering the object to object_entry array. The first one moves the call site to the hash function a bit, to give add_object_entry() to inspect the pathname.

The second patch teaches add_object_entry() to record if an entry should be exempt from the deltification process, and use the .gitattributes mechanism to tell which objects should be marked as such.

The patches are based on np/pack series, and should apply cleanly to 'next'.