Visualize the expected contribution of a child branch to its parent

[mdel]

Hello,

I wish to have an option from the contextual menu to see the expected contribution of a child branch to its parent (if merged), which is the diff between the tip of the branch and the common ancestor of the main branch (F0/A0 and H1/C1 in the diagram below)

Today if I used the "diff branch" feature, it shows me the diff from the origin of the branch. If I take the /main1 example below, "diff branch" shows H1/A1 instead of H1/C1, as a result the "diff branch" contains a lot of changes which actually are not part of /feature1 (here it will be C1 and B1 changes).

I can manually visualize what I want by manually selecting the diff between H1 and C1, but it big repository it is a tedious operation and a which for automated button to show that diff.