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
#require serve
$ hg clone http://localhost:$HGPORT/ copy
abort: * (glob)
[100]
$ test -d copy
[1]
$ "$PYTHON" "$TESTDIR/dumbhttp.py" -p $HGPORT --pid dumb.pid
$ cat dumb.pid >> $DAEMON_PIDS
$ hg clone http://localhost:$HGPORT/foo copy2
abort: HTTP Error 404: * (glob)
[100]
$ killdaemons.py