git-delta 0.19.2

#!/bin/bash
REPO_ROOT=/tmp/submodule-diff-example
REPO_DATE_FMT=%H:%M:%S.%N
mkdir ${REPO_ROOT} && git -C ${REPO_ROOT} init && cd ${REPO_ROOT}
date +${REPO_DATE_FMT} >> baserepo && git add baserepo && git commit -m "Base repo commit 1"

for sub in A B C; do
    git init submodule${sub}
    for iter in $(seq 1 4); do
        date +${REPO_DATE_FMT} >> submodule${sub}/subcontent
        git -C submodule${sub} add subcontent && git -C submodule${sub} commit -m "Submodule ${sub} initial commit $iter"
    done
    # Add initial submodule, message of "Submodule submoduleX 0000000...xxxxxxx (new submodule)", no individual commits
    git submodule add ../bogus-url-${sub} submodule${sub} && \
        git commit -m "Add submodule${sub}" # the diff handling for this is correct in delta

    # Create additional submodule commits
    for iter in $(seq 1 2); do
        date +${REPO_DATE_FMT} >> submodule${sub}/subcontent
        git -C submodule${sub} add subcontent && git -C submodule${sub} commit -m "Submodule ${sub} extra change ${iter}"
    done
    git add submodule${sub} && git commit -m "Update submodule${sub}"
done

git -C submoduleA reset --hard HEAD~4
git -C submoduleC reset --hard HEAD~2

for sub in B C; do
    for iter in $(seq 1 3); do
        date +${REPO_DATE_FMT} >> submodule${sub}/subcontent
        git -C submodule${sub} add subcontent && git -C submodule${sub} commit -m "Submodule ${sub} stage change ${iter}"
    done
done

# Add all submodule updates in single commit to test multiple submodule updates in single commit
git add submodule[A-C] && git commit -m "Update all submodules"

# submoduleA end state is only removed commits
# submoduleB end state is only added commits
# submoduleC is a mixture of removed and added commits (e.g. different branch)

# Manual, inspect superproject history via:
#    "git -c diff.submodule=short log -p | delta --no-gitconfig"
#    "git -c diff.submodule=log log -p | delta --no-gitconfig"
#    "git -c diff.submodule=diff log -p | delta --no-gitconfig"