Make neovim deploy more robust

[pokey]

Today, neovim deploy assumes that any changes to the staging dir come from the head commit. But if there is a race and two commits come in at nearly the same time due to merge queue, we could attribute the changes to the wrong commit

With cursorless-talon it works well because we use a subtree split, but we can't do that here because deploy is not simply a subtree: we actually build something

I'm not sure exactly the best way to do this. One way would be to keep a staging branch of some sort similar to how we do for cursorless-talon, and then re-build and create the staging directory for every commit between last deployed and head. I still think this might result in a race. We should really try to ensure determinism: the commit history on monorepo main completely determines the commit history on deploy branch