[SPARK-9179] [BUILD] Allows committers to specify primary author of the PR to be merged

It's a common case that some contributor contributes an initial version of a feature/bugfix, and later on some other people (mostly committers) fork and add more improvements. When merging these PRs, we probably want to specify the original author as the primary author. Currently we can only do this by running

```
$ git commit --amend --author="name <email>"
```

manually right before the merge script pushes to Apache Git repo. It would be nice if the script accepts user specified primary author information.

Author: Cheng Lian <lian@databricks.com>

Closes #7508 from liancheng/spark-9179 and squashes the following commits:

218d88e [Cheng Lian] Allows committers to specify primary author of the PR to be merged