dirstate-v2: add check of parent/child nodes being related when writing This stems from a corruption seen in a private repository. We're not sure of the source of the corruption, and it's very possible that we're seeing compounded effects of multiple writes on a corrupted dirstate. Adding this check is not expensive in itself and large writes of the dirstate are not common. This change does not catch this problem at the root node, the next one will.

admin-verify: expect a number of errors to be returned It's the responsibility of the check to handle errors, we only care about the total count to sum up the check's work. We use `admin::verify -c dirstate` to test this path at least somewhat.

admin-verify: fix error message handling `dirstate.verify` used to return tuples but does not anymore, it returns the pre-formatted error message, which is a nicer interface anyway.

admin-verify: pass p1 down to the dirstate function This was forgotten and can break with certain kinds of corruption.