Use comments rather than statuses to save metadata
Show pull request numbers in the merge status
Consider only the latest status when checking if a commit is successful
Test a rollup again if one of the PRs advances, without the advanced PR
Reduce the number of runs needed to merge a successful pull request

Basically, we can skip the TESTED state entirely. We can also check the
next pull request at the same time.

Fixes #35.
Do not manually close a pull request

Because of the manual closing, the pull request is occasionally set to
"closed" rather than "merged" even though it was successful. This is
because there is a little latency between the fast-forwarding and the
automatic transition to "merged".

If a bad thing happens, it would be better to leave it "open".
Parse the commands more generously

r+, r=me, r=[name], r-, and retry are checked more generously.