Worktree-aware ncu config file resolution

[mmarchini]

I use git worktree a lot, it would be nice if node-core-utils could identify when we're running commands on a worktree and look for .ncu/ on the main worktree.

[targos]

We'd need to have some special cases because config.upstream makes sense to be shared between worktrees but not config.branch.

[mmarchini]

That makes sense, although in my use case last week I was using the same branch name :)

[github-actions[bot]]

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