Customise entire merge commit comment

Problem Definition

When merging a pull request, the merge commit comment always begins with a generic message, e.g.

Merge pull request #4 in PROJECT/myrepo from feature/feature_a to master

This is not always desirable, for example when developers make use of the git log --oneline command to view merge history, because the first line of the merge commit comment can't be set

Suggested Solution

Allow the entire merge commit comment to be customised, so that user comments can be placed at the beginning