Update merge_pr_body to handle new PR merges

[shirakia]

Motivation

Merging PR body seems to be buggy when old_body is modified.

Current

x-motemen#21

Proposal

Show additional PR like

- [ ] #1 Init new func @test_user
- [ ] #2 Add new func @test_user

In this PR, we've done blah blah blah ...

When we release, we should do blah blah blah ...

Additional PRs

- [ ] #3 Fix bugs @test_user
- [ ] #4 Add more func @test_user
- [ ] #5 Refactor a bit @test_user
- [ ] #6 hogehoge @test_user"

Approach

To get PR body diff, just use Array -.