Rename an existing reference

This method works for both direct and symbolic references. The new name will be checked for validity and may be modified into a normalized form.

The given git_reference will be updated in place.

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.