Improve usefulness of per-commit statuses

Instead of creating -commit-check statuses for all commits in a topic, instead create a full commit status for each as if it were the last commit in a topic. Basically, each commit should be checked as if it were the HEAD of the topic.

However, upon trying to implement this, it is not as easy as we thought when discussing offline. Major blockers:

• A naïve comment_text(parent_res + parent_res + self_res) may result in duplicate error messages if the parents share a commit with an error;
• topic-level checks are not run on each commit, so checks which might reject if it were the head commit don't run on those commits which means that they are not full branch checks.

Instead, I propose we just drop them. Other services seem to not submit statuses for intermediate commits anyways.

Cc: @brad.king