compactionHeuristic=true is not used by interactive staging

Hello, I just did a fresh clone of git from Github and installed the version 2.9.0 on Fedora 22.

I tried the new compactionHeuristic = true, which is awesome. The only thing that struck me was that this option was not used when doing an interactive staging, meaning `git diff` and `git add -p` will format patches differently. Perhaps this is intended and there is a way to force interactive staging to use specific diff options, but I did not find it in the doc.

Thanks, Alex