Consider not commenting about new commits if the last comment was made by highfive

[emilio]

I feel bad for spamming people when pushing little fixups :-)

The worst case, that could be a needed rebase, is addressed by bors comment about the bitrot.

[jdm]

This would be nice, but I suspect it would incur an extra API request to check the comments each time there's a push. I'm pretty sure we're already hitting the upper limit during really active periods :(