This is due to using the actual merge code for generating the preview diff. When you merge, all the revisions involved in the merge are copied into the repository of the branch that you're merging into. Changing this would be nontrivial.
This is due to using the actual merge code for generating the preview diff. When you merge, all the revisions involved in the merge are copied into the repository of the branch that you're merging into. Changing this would be nontrivial.