read-tree: teach 1-way merege and plain read to prime cache-tree.

read-tree: teach 1-way merege and plain read to prime cache-tree.

This teaches read-tree to fully populate valid cache-tree when
reading a tree from scratch, or reading a single tree into an
existing index, reusing only the cached stat information (i.e.
one-way merge).  We have already taught update-index about cache-tree,
so "git checkout" followed by updates to a few path followed by
a "git commit" would become very efficient.

Signed-off-by: Junio C Hamano <junkio@cox.net>