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.
$ hg init repo
$ cd repo
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls -A
.hg
a
unknown