git-stash: try reusing cached stat info as much as possible

git-stash: try reusing cached stat info as much as possible

Earlier when we read a tree into a temporary index, we read it
from scratch.  Start from the current index and use read-tree -m
to preserve cached stat information as much as possible, in
order to speed up "git add -u".  This makes "git stash" usable
in a source tree of nontrivial size.

Signed-off-by: Junio C Hamano <gitster@pobox.com>