debugtagscache: verify that filenode is correct

Previous patch from Matt demonstrates that `debugtagscache` does not warn about
filenode being unknown which can be caused by a corrupted cache.

We start by showing that it's an unknown node.

Differential Revision: https://phab.mercurial-scm.org/D10015
13 jobs for topic/default/tags in 31 minutes and 8 seconds (queued for 4 seconds)
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