Currently, Bitbucket doesn’t allow you to create a pull request against a tag. While I understand that is not really the function of tags, it means effectively that we must create pull requests on the head of branches (instead of tags).
In turn, this means that any branch that is to be submitted must be created from the head of a branch, not a tag (making the workflow of creating a branch based off of a tag endorsed by the apple git tools very suboptimal).
If you create a branch from a tag, you can easily run into the situation where you see other contributor’s commits in the pull request.

This is an ER for allowing people to diff against a tag instead of a branch head on Bitbucket, even if the submission will go into the branch (not the tag).