Revisions walker: Add notification for truncated/shallow histories

It might exists some rare cases where a revision's parent is referenced but doesn't exist in the archive which results in a truncated/shallow history when walking on the revisions graph.

Changes introduced in !904 (closed) will guarantee that revision walkers will not end up in error when encountering such cases.

Nevertheless, being notified that the returned history is invalid would be of interest. We should find a way to implement that behavior.

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Migrated from T1677 (view on Phabricator)