Not sure why you are being downvoted. You actually can use merge commits this way, by viewing the diff produced by git diff $merge_commit^...$merge_commit^

Not sure why you are being downvoted. You actually can use merge commits this way, by viewing the diff produced by git diff $merge_commit^...$merge_commit^2 when interested in whole change-set introduced by a branch or looking at individual commmits when interested in well, individual commits.