git-apply: be a lot more careful when writing files

git-apply: be a lot more careful when writing files

We write them under another name and rename them to their destination,
so that if something bad happens in the middle, we won't have caused any
bigger harm.

Also, this makes the writing be NFS "intr" safe, and as a side effects
makes sure that if the target is hardlinked (or symlinked) we will have
broken the link.