PARQUET-1779: Update merge script

[xhochy]

This script was already used to merge #157

Jira

• My PR addresses the following Parquet Jira issues and references them in the PR title. For example, "PARQUET-1234: My Parquet PR"
  • https://issues.apache.org/jira/browse/PARQUET-1779
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Commits

• My commits all reference Jira issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain Javadoc that explain what it does

[Fokko]

With the gitbox integration, I don't use these scripts anymore. However, I don't mind if someone prefers using these scripts.

[xhochy]

I like these scripts as they automate the JIRA work away.

[pitrou]

I'll note that the Arrow script that is being copied here received some later changes, but none seems mandatory for proper functionality.

$ git log --format=oneline  --after=2020-01-28 dev/merge_arrow_pr.py
5eef8c399c9d5fa5f15289a5e12bd222135c6797 ARROW-11941: [Dev] Don't update Jira if run "DEBUG=1 merge_arrow_pr.py"
28bd4da384786673caac0244a4156334f7331264 ARROW-6915: [Developer] Do not overwrite point release fix versions with merge tool
5ca82175e9c06b1f3308fb07544c3f4812b613fa ARROW-7530: [Developer] Do not include list of PR commits in commit message when using PR merge tool
5ffbf0ab3a2bb085b76e55e5cb862c2a74ade552 ARROW-8016: [Developer] Fix jira-python deprecation warning in merge_arrow_pr.py

@xhochy Do you want to synchronize this PR anyway or should we simply merge the current version?