x/build/cmd/gopherbot: use the commit message if there's exactly one

[mvdan]

Time and time again, I see contributors updating their commit messages on their PRs, instead of updating the PR title and body. I can understand that - it's more intuitive, it can be done very easily with git, and it mirrors what people do with Github and Gerrit.

However, gopherbot does the opposite - it ignores the actual commit messages, but uses the PR title and body. This is can be good for newcomers, especially those who aren't used to squashing commits.

The result is that we end up wasting time every week, when contributors get stuck fighting gopherbot. This just happened again: https://go-review.googlesource.com/c/go/+/126897

I propose that the PR title and body be only used if there are many commits. Otherwise, the single's commit message should be used.

cc @andybons @bradfitz @dmitshur

[mvdan]

I just realised this should be a comment on #25359, but I failed to find the issue. Will close and comment there.