R1:b3ae48d8ca2d

Improve Differential query plans

Summary:
Ref T8575. We run a big "(A) UNION (B)" query on the home page and on the main Differential page.

"A" can always be improved by using `%Ls`, so it can use the second half of the `(authorPHID, status)` key.

"B" can sometimes be improved if the fraction of open revisions is smaller than the fraction of revisions you are reviewing. This is true for me on secure.phabricator.com (I'm a reviewer, either directly or via 'Blessed Reviewers', on about 80% of revisions…