Move 'unpack_trees()' over to 'traverse_trees()' interface

This not only deletes more code than it adds, it gets rid of a
singularly hard-to-understand function (unpack_trees_rec()), and
replaces it with a set of smaller and simpler functions that use the
generic tree traversal mechanism to walk over one or more git trees in
parallel.

It's still not the most wonderful interface, and by no means is the new
code easy to understand either, but it's at least a bit less opaque.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>