Fix an issue where recurring ghost events could go missing if queried with a limit

Summary:
Ref T11816. Depends on D17644. When you executed a query like "upcoming, limit 5 events" you might match some recurring events starting from, say, a year ago and repeating every month.

We'd then generate the first 5 ghosts for these events (say, last January, February, ... May) and later throw them out, so the correct events in the query window (say, this April) would never get generated.

Instead, generate ghosts beginning with the start of the window. The fix in D17644 to number results correctly allows us to do this.

Test Plan:

• Made a query panel showing 5 events, scheduled an event long in the past, did not visit any of the instances of it so they didn't generate concrete objects.
• Before the patch, near-future instances failed to show; after the patch, they show.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D17645