Merge branch 'jh/unpack-trees-micro-optim'

In a 2- and 3-way merge of trees, more than one source trees often
end up sharing an identical subtree; optimize by not reading the
same tree multiple times in such a case.

* jh/unpack-trees-micro-optim:
unpack-trees: avoid duplicate ODB lookups during checkout

diff --cc unpack-trees.c
Simple merge