Allow unrelated histories when rebasing to master (#29699)
Summary:
Some prs are refused to be merged.
For example, https://github.com/pytorch/pytorch/pull/29595
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29699
Reviewed By: hl475
Differential Revision: D18473531
Pulled By: houseroad
fbshipit-source-id: e7a4eb1b4be9d9da6dc281575eeb4d7ae685b531
