Git - easiest way to see diff with previous version if I have the sha

If you want to view the diff visually in kdiff3, meld, kompare, xxdiff, tkdiff, diffuse…

git difftool --dir-diff shaOfHisCheckIn^!

git difftool --tool=meld --dir-diff shaOfHisCheckIn^!

git difftool -t meld -d shaOfHisCheckIn^!

git show is your friend:

git show shaOfHisCheckIn