fix repack with --max-pack-size

fix repack with --max-pack-size

Two issues here:

1) git-repack -a --max-pack-size=10 on the GIT repo dies pretty quick.
   There is a lot of confusion about deltas that were suposed to be
   reused from another pack but that get stored undeltified due to pack
   limit and object size doesn't match entry->size anymore.  This test
   is not really worth the complexity for determining when it is valid
   so get rid of it.

2) If pack limit is reached, the object buffer is freed, including when
   it comes from a cached delta data.  In practice the object will be
   stored in a subsequent pack undeltified, but let's make sure no
   pointer to freed data subsists by clearing entry->delta_data.

I also reorganized that code a bit to make it more readable.

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