I'm not sure this would fix the command line length, it it just moves the length around.
I've been thinking of a real fix for this, and I think it needs to put it all in a temporary file, kinda

.   for d in ${DISTFILES}
     printf "%s\n" "$d" >> temp
.   endfor

and then pass maybe either pass the tempfile name to do-fetch, or use it as its stdin.

Also, there is the same problem for each script that we pass the distfiles to, like makesum.

Maybe using \0 instead of \n is better there, and juggling with IFS to split on \0, in case some distfile has a strange file name.

yesh I pondered the tempfile, but this was longer, actually, but yes I think you are right, as printf will complain also about the ARG_MAX.

makesum will need a re-design, thinking about it.

I may have "fixed" makesum, but maybe not, and I do not seem to find a port to test things.