I'm using GNU diff 3.7 and zsh 5.7.1 as provided by Debian 10.2, and I noticed that zsh's completion doesn't support diffs -w/--ignore-all-space parameter. This is pretty annoying, because it means that it won't complete "new file" in this case:

diff -uw old new<TAB>

It'll think that -uw is "old file", and that "old" is "new file". I don't fully understand the _diff_options completion script, so my change might probably not be completely correct, but the attached patch fixes it for me.

Martin