Support renaming the `master` branch without disruption

To go along with https://jira.atlassian.com/browse/BCLOUD-20212 , I'd like to request support for renaming the `master` branch of an existing repository without causing massive disruption. For example:

• Existing local clones that reference the old branch name should still be able to fetch it and (if possible) push to it, possibly with a warning to the user. There's an ongoing discussion of features that might be added to the standard git client and server to facilitate branch renames.  BitBucket may be able to use some of those or may need its own implementation depending on technical constraints.
• Existing pull requests targeting the old branch name should be updated to target the new branch name.
• Forks of the repository hosted on BitBucket or other things on BitBucket may need to be updated in some way; someone familiar with all the features would have to think this through.

It's possible to manually create a branch of the new name and change the default branch, but that doesn't do any of the things above.

See:

• GitHub's brief plan for a "seamless move for existing repositories"
• Analogous feature request for GitLab