SPARK-4507: PR merge script should support closing multiple JIRA tickets

This will fix SPARK-4507.

For pull requests that reference multiple JIRAs in their titles, it would be helpful if the PR merge script offered to close all of them.

Author: Takayuki Hasegawa <takayuki.hasegawa0311@gmail.com>

Closes #3428 from hase1031/SPARK-4507 and squashes the following commits:

bf6d64b [Takayuki Hasegawa] SPARK-4507: try to resolve issue when no JIRAs in title
401224c [Takayuki Hasegawa] SPARK-4507: moved codes as before
ce89021 [Takayuki Hasegawa] SPARK-4507: PR merge script should support closing multiple JIRA tickets