pack-objects: free preferred base memory after usage

pack-objects: free preferred base memory after usage

When adding objects for preferred delta base, the content from tree
objects leading to given paths is kept in a cache. This has the
potential to grow significantly, especially with large directories as
the whole tree object content is loaded in memory, even if in practice
the number of those objects is limited to the 256 cache entries plus the
$window root tree objects.  Still, that can't hurt freeing that up after
object enumeration is done, and before more memory is needed for delta
search.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>