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>

git-stash.sh

@@ -58,11 +58,11 @@ save_stash () {
 
 	# state of the working tree
 	w_tree=$( (
+		rm -f "$TMP-index" &&
+		cp -p ${GIT_INDEX_FILE-"$GIT_DIR/index"} "$TMP-index" &&
 		GIT_INDEX_FILE="$TMP-index" &&
 		export GIT_INDEX_FILE &&
-
+		git read-tree -m $i_tree &&
-		rm -f "$TMP-index" &&
-		git read-tree $i_tree &&
 		git add -u &&
 		git write-tree &&
 		rm -f "$TMP-index"