Regarding using `git rebase --autosquash`, in my experience, it's much simpler to just use 'git reset <first commit>' and then [git commit --amend](https://www.atlassian.com/git/tutorials/rewriting-history). Just be sure to avoid using using 'git reset --hard' in that case! For example, if you have 4 commits on a feature branch, and you want the last 3 'squashed' into the first one, you can use 'git reset <1st commit>', then use 'git commit --amend ...' (see git docs for complete usage).
I have serious doubts that most regular git users or even maintainers are familiar with using rebase with autosquash and the --fixup directive. It seems a lot more trouble than it's worth for a feature branch (especially when maintainers can view a unified diff in any case). I'll concede that for those who have learned how to use it, it's probably no big deal. I first tried it a few weeks ago, and after a few tries, managed to squash my last commit into my 3rd to last commit. Last night I tried it, and somehow lost 20 minutes of work.
But I'm closing this PR now, since it's 3 years old and apparently not wanted, nor was it ever requested by the Geany maintainers.