git-read-tree: be a lot more careful about merging dirty trees

We don't want to overwrite state that we haven't committed yet
when merging, so it's better to make git-read-tree fail than
end up with a merge tree that ends up not having the dirty changes.

Update git-resolve-script to fail cleanly when git-read-tree fails.

git-resolve-script

@@ -39,13 +39,14 @@ if [ "$common" == "$head" ]; then
 	echo "Destroying all noncommitted data!"
 	echo "Kill me within 3 seconds.."
 	sleep 3
-	git-read-tree -m $merge && git-checkout-cache -f -u -a
+	git-read-tree -m $merge || exit 1
+	git-checkout-cache -f -u -a
 	echo $merge > "$GIT_DIR"/HEAD
 	git-diff-tree -p ORIG_HEAD HEAD | git-apply --stat
 	exit 0
 fi
 echo "Trying to merge $merge into $head"
-git-read-tree -m $common $head $merge
+git-read-tree -m $common $head $merge || exit 1
 merge_msg="Merge of $merge_repo"
 result_tree=$(git-write-tree  2> /dev/null)
 if [ $? -ne 0 ]; then