My work flow:
For CODE REVIEW, I need to show the differences between two heads and squash/organize my commits ( in about 5 commits ). What's the best GUI (cross-platform?) for this task?
The Sourcetree free Git GUI for Windows and Mac supports this.
Alternatively, to do it without a GUI, you can run
git rebase --interactive --autosquash
because you committed with commit message beginning with !squash (when those intermediate commits were about the same task)
See "Trimming GIT Checkins/Squashing GIT History".