typing: declare the `_phasesets` member of `phasecache` to be `Optional`
Something in this area got flagged while making the repository class visible to
pytype (instead of being typed as `Any`). A None assignment to something not
optional is wrong, and when I tried setting it to `{}` to keep it non-Optional,
some tests failed. There are checks for the attr being None elsewhere, so this
seems to have just been an oversight.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state