### What changes were proposed in this pull request?
Introduce PR merge script `dev/merge_pr.py`, which is borrowed from Apache Spark
### Why are the changes needed?
This script simplifies the PR merge procedure
- auto backport to release branches
- auto close the JIRA ticket
- auto fill in the JIRA fixed version
- reserve the PR description in git log
- reserve the author and committer in git log
### Does this PR introduce _any_ user-facing change?
No, it's for committers.
### How was this patch tested?
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|---|---|---|
| .. | ||
| checkout_pr.sh | ||
| merge_pr.py | ||
| reformat | ||