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

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.