.arcconfig

dirstate: invalidate changes when parent-change fails

When an error occurs during changing parents, we should invalidate all dirstate
modifications and reload the dirstate. This is currently done by a `unlock`
callback on the `wlock`.

To fix this anomaly, we start dealing with the error directly in the context
manager and its potential nesting.

The "hard" part is to make sure that, when the parent-change context are nested,
we and higher level nesting do not continue to use the invalidated dirstate.
We introduce dedicated code to enforce that.

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