Do you ever really want this? I don't recall wanting this. But you can still get this: just list the ${base_ref}..${deb_ref} commit ranges, select the commit you want, and diff the `git show` of the selected commits. It helps here to keep the commit synopsis the same.

E.g.,

  c0=$(git log --oneline ${base_ref0}..${deb_ref0} |
         grep "^[^ ] The subject in question" |
         cut -d' ' -f1)
  c1=$(git log --oneline ${base_ref1}..${deb_ref1} |
         grep "^[^ ] The subject in question" |
         cut -d' ' -f1)
  if [[ -z $c0 || -z $c1 ]]; then
    echo "Error: commits not found"
  else
    diff -ubw <(git show $c0) <(git show c1)
  fi

See also the above commentary about Gerrit and commit IDs.

(Honestly I don't need commit IDs. What happens if I eventually split a commit in a patch series into two? Which one, if either, gets the old commit ID? So I just don't bother.)