One could accomplish a part of your vision, today, by splitting commits into smaller commits. Then you have the relation between hunks of changes and text in the commit message on a smaller level. Then you can use branches with a non-trivial commit message in the merge commit to document the whole set of commits.

As far as I know changes to the Linux kernel are usally submitted as a series of patches, i.e. a sequence of commits. I.e. a branch, although it is usually not represented as git branch while submitting.