Rename an existing reference.

This method works for both direct and symbolic references.

The new name will be checked for validity. See git_reference_create_symbolic() for rules about valid names.

On success, the given git_reference will be deleted from disk and a new git_reference will be returned.

The reference will be immediately renamed in-memory and on disk.

If the force flag is not enabled, and there's already a reference with the given name, the renaming will fail.

IMPORTANT: The user needs to write a proper reflog entry if the reflog is enabled for the repository. We only rename the reflog if it exists.