the existing diff calculation approach directly compares branches, this forces linear history for merge. this patch updates the diff logic to use the common ancestor commit to calculate diffs. ``` P Q o---o topic / / o---o---o---o master A X Y ``` when diffing topic and master directly with difftree (Q and Y), the commits introduced in master since the creation of the branch, namely, X and Y, are considered "removed" from the generated patch. however, when diffing with the common ancestor A, only changes introduced by P and Q are included in the produced patch. this calculation can be retired in favor of format-patch soon enough (i think).