>I suppose I have to fix the ".rej" file and reapply it; but do you fix it with a text editor or by regenerating the patch with the `make update-patches` workflow? That's not clear to me.
I'd assume you'd have to mv <file>.orig <file> and fix the patch file 
itself either with rediff(1) (from textproc/patchutils) or manually, 
ditching the .rej file, however I'm not an expert either

>Can you believe it's the first time I ever had to use patch(1) and diff(1)? I'll figure it out soon.
There's a first time for everyone :)