tags: behave better if a tags cache entry is partially written
This is done by discarding any partial cache entry, instead of
filling the partial cache entry with 0xff before.
Differential Revision: https://phab.mercurial-scm.org/D8095
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state