ARROW-6262: [Developer] Show JIRA issue before merging
ARROW-6262: [Developer] Show JIRA issue before merging

It's useful to confirm whehter the associated JIRA issue is right or
not.

We couldn't find wrong associated JIRA issue after we merge
the pull request https://github.com/apache/arrow/pull/5050 .

Closes #5097 from kou/dev-merge-show-jira-issue-before-merge and squashes the following commits:

6c9ad5be9 <Sutou Kouhei>  Show JIRA issue before merging

Authored-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>
Signed-off-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>

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