Issue commit comments are processed again when merging (BB-11792)

Under certain conditions, commit issue comments may appear a second time when merging a pull request. The person who merged the pull request will appear as the author of this comment.

An issue shouldn't be transitioned again, nor should there be a message, if the issue is already in the transition status.

The person shown to have performed the action is technically accurate as we log all comments to the person who either pushed the commit containing the message, or the person who performed the action on the Bitbucket site.