Maestro Bot: Branch merge PRs use Git "committer" field when listing users who contributed changes to the merge

Migrated from https://github.com/dotnet/core-eng/issues/8070

@anurse wrote:

Example: https://github.com/aspnet/AspNetCore/pull/14714

This PR mentioned me as the person who had commits involved in this merge. That's somewhat correct, as I had to push the merge button on behalf of the original author (@Tratcher in this case) since the branch was protected and required an admin to merge. However, the commits themselves were authored by @Tratcher and he would have more context on how to resolve conflicts.

My suggestion is that the merge bot either:

1. Use the "author" field to build this list. Generally it's the same as the committer, unless there's a situation like this. It should always represent the user who originally created the commit

1. Report both "author" and "committer" in this list. I certainly don't *mind* being mentioned but it would save time to have the author also be mentioned.