Consider using the title of merge commits to generate changelogs

[nicholasyang2022]

We have been using pull request title and description as the commit message of merge commits. Pull request titles should ocus more on the features delivered instead of implementation details, so that they should be more suitable for changelog. And implementing this will freeing us from removing doc/tests related commits from changelogs manually.

obs service tar_scm is not able to do this. We may need to disable the changesgenerate feature of tar_scm and use a standalone script to generate changes in this way.