The default value for a merge commit message.- `PR_TITLE` - default to the pull request's title.- `PR_BODY` - default to the pull request's body.- `BLANK` - default to a blank commit message.

Required when using `merge_commit_message`.The default value for a merge commit title.- `PR_TITLE` - default to the pull request's title.- `MERGE_MESSAGE` - default to the classic title for a merge message (e.g., Merge pull request #123 from branch-name).

The default value for a squash merge commit message:- `PR_BODY` - default to the pull request's body.- `COMMIT_MESSAGES` - default to the branch's commit messages.- `BLANK` - default to a blank commit message.

Required when using `squash_merge_commit_message`.The default value for a squash merge commit title:- `PR_TITLE` - default to the pull request's title.- `COMMIT_OR_PR_TITLE` - default to the commit's title (if only one commit) or the pull request's title (when more than one commit).