tags: validate nodes in _getfnodes() and update cache in case of unknown nodes `hgtagsfnodescache` can contain unknown nodes due to cache corruption and this lead to a traceback on operations like `hg tags` as we don't validate nodes. This patch validates that all filenodes returned after `hgtagsfnodescache` are known to the repository. If there exists any unknown filenode, we force recompute it and update the cache. The test change demonstrates the fix. Differential Revision: https://phab.mercurial-scm.org/D10083