Fix merge PR title to be followed with proper new line

Now, merge pull-request makes a commit like:

    Merge pull request #8 from yasuoza/blahblah\n\nOriginal Pull Request Title

This commit fixes it to:

    Merge pull request #8 from yasuoza/blahblah

    Original Pull Request Title
Merge pull request #617 from yasuoza/gh-fix_merge-message

gh: Fix merge PR title to be followed with proper new line