read-tree() and unpack_trees(): use consistent limit

read-tree() and unpack_trees(): use consistent limit

read-tree -m can read up to MAX_TREES, which was arbitrarily set to 8 since
August 2007 (4 is needed to deal with 2 merge-base case).

However, the updated unpack_trees() code had an advertised limit of 4
(which it enforced).  In reality the code was prepared to take only 3
trees and giving 4 caused it to stomp on its stack.  Rename the MAX_TREES
constant to MAX_UNPACK_TREES, move it to the unpack-trees.h common header
file, and use it from both places to avoid future confusion.

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