x/build/cmd/gerritbot: should determine author from first commit rather not last

[dmitshur]

When an imported PR has many commits, the Gerrit CL author is seemingly determined by taking the git author of the latest commit in the PR.

I think it may be better to take the git author of the first commit rather than last. That way, if an additional contributor adds more commits, the author of the CL would not change. The first commit seems like a more stable source for this information.

I noticed this in https://go-review.googlesource.com/c/go/+/216617/47..48 when I pushed a new merge commit to the end of https://github.com/golang/go/pull/36815/commits, and the author of the Gerrit CL changed.

/cc @golang/osp-team @griesemer

P.S. For bonus points, it can be considered to convert additional authors into "Co-authored-by" lines.