The name "null" is confusing; you have to pick something. However, I think what is desired here is the "theirs" strategy, i.e. to replace the current branch's tree entirely with the incoming branch's tree. The end result would be similar to a hard reset onto the incoming branch, except that it would also create a merge commit. Unfortunately, the "theirs" strategy does not exist, even though the "ours" strategy does exist, apparently to avoid confusion with the "theirs" option [1], but it is possible to emulate it with a sequence of commands [2].

[1]: https://git-scm.com/docs/merge-strategies#Documentation/merg...

[2]: https://stackoverflow.com/a/4969679/814422