selective git-update-index per diff(1) chunks

Pretty often I end up with a file with two simple orthogonal fixes in it. git-diff shows me both, that's OK.

Now I want to commit them as two separate commits. So far, it's
* getting full diff
* cp(1)
* hand-edit both diffs
* commit first
* commit second

Has anyone thought about aggregating this into git-update-index or somewhere?

    git-update-index -C1,3    #chunks 1, 3
    git commit
    git-update-index -C1,3    # chunks 2,5 in original numbering
    git commit