Investigate new `merge-tree --write-tree` flag
git merge-tree
now supports --write-tree
which inherently avoids touching the worktree in Git 2.38.0.
https://github.blog/2022-10-03-highlights-from-git-2-38/
Not sure if it would simplify anything here as we still want our smarter submodule conflict resolution logic.
Cc: @brad.king