Change default message of merge commit to PR tilte and description

[eitsupi]

Currently, the merge commit message contains a list of messages from the squashed commit.
But given that the squashed commits may contain ultimately irrelevant material that was changed along the way, I think that PR's description would often be more intelligible to a later reader than commit message list.

So I am wondering if it is worth changing the commit message to PR tilte and description.

[max-sixty]

+1 from me...

[max-sixty]

I switched it; we can reopen if anyone disagrees...