Sure, you can use git to do this, but the git killer will have it as an expected capability.

I also think that octopus merges are basically always a disaster because they can't be meaningfully reviewed and put your repo into an unknown state. Maybe there's some way to get the advantages of merge commits (preserve all history!) without the disadvantages (jumble all history!).

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.

Yup, and you can use --first-parent to git bisect, git log, git praise to interact at the "macro-level" of those merge commits by default, and dive in to the fuller graph only as necessary.