read-tree: teach 1 and 2 way merges about cache-tree.

This teaches one-way and two-way "read-tree -m" (and its special
form, "read-tree --reset" as well) not to discard cache-tree but
invalidate only the changed parts of the tree.  When switching
between related branches, this helps the eventual commit
(i.e. write-tree) by keeping cache-tree valid as much as
possible.

This does not prime cache-tree yet, but we ought to be able to
do that for no-merge (i.e. reading from a tree object) case and,
and also perhaps 1 way merge case.

With this patch applied, switching between the tip of Linux 2.6
kernel tree and a branch that touches one path (fs/ext3/Makefile)
from it invalidates only 3 paths out of 1201 cache-tree entries
in the index, and subsequent write-tree takes about a half as
much time as before.

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

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