Search backend: use efficient implementation for repo:contains.commit.after()

Created by: camdencheek

This just plugs the repo:contains.commit.after() predicate into the more-efficient machinery we use for repohascommitafter.

A little more on direction here: I think I'm going to make it my near-term goal to completely remove the Plan() method from the Predicate interface. The method is very restrictive in that 1) it requires the result to be representable as a search query fragment, and 2) it requires the predicate to be evaluated in advance. Getting rid of Plan() also lets us get rid of the awkward "expand predicates" step which breaks us out of our very nice pattern of "create a plan, execute a plan."

Test plan

Unit tests, integration tests, and light manual tests.