This very specific bash script is used to amend commits in a git tree and then rebase all affected branches and move all affected tags to the new commit. Crazy and dangerous, but better than doing it manually all the time.

git_replace_commit

#!/usr/bin/env bash

set -e

OLD_BASE=$1 # v1.0.0-step-1
ADDITION=$2

N=$'\n'
ME="$( basename ${BASH_SOURCE[0]} )"
DRY_RUN=false
VERBOSE=true

function echoAndEval() {
  if [ "${VERBOSE}" = true ]; then
    echo "$@"
  fi

  if [ "${DRY_RUN}" = false ]; then
    eval "$@"
  fi
}

function moveChildren() {
  for CHILD in "${!WITH_PARENT[@]}"; do
    if [ "${WITH_PARENT[$CHILD]}" = "$1" ]; then
      OLD_BRANCH=$(echo ${CHILD}| cut -d'/' -f 2)
      NEW_BRANCH=${OLD_BRANCH}_on_$2

      echo "$N[$ME] Creating ${NEW_BRANCH} from ${OLD_BRANCH} on $3."

      echoAndEval git checkout -b ${NEW_BRANCH} ${CHILD}
      NEW_BRANCHES[${#NEW_BRANCHES[*]}]=${NEW_BRANCH}

      set +e
      echoAndEval git rebase --committer-date-is-author-date --preserve-merges --onto $3 $(git_parent_commit ${CHILD})
      set -e

      if [ "${DRY_RUN}" = false ]; then
        while [ "$(git status | grep 'interactive rebase in progress' | wc -l)" -gt "0" ]; do
          echoAndEval git rebase --skip
        done
      fi

      DESTRUCTIVE+=("git push --force origin ${NEW_BRANCH}:${OLD_BRANCH}")
      CLEANUP+=("git branch -d ${NEW_BRANCH}")

      moveChildren ${CHILD} ${NEW_BASE} ${NEW_BRANCH}
    fi
  done
}

BRANCHES=$(git branch -r --contains ${OLD_BASE})
TAGS=$(git tag --contains ${OLD_BASE})

ROOTS=()
declare -A WITH_PARENT=()

while read -r CURRENT; do
  PARENT="$CURRENT"
  DIST="1000000"
  while read -r CANDIDATE; do
    CHILD_DISTANCE="$(git rev-list --count $(git merge-base ${CURRENT} ${CANDIDATE})..${CURRENT})"
    PARENT_DISTANCE="$(git rev-list --count $(git merge-base ${CURRENT} ${CANDIDATE})..${CANDIDATE})"
    if [ "${CHILD_DISTANCE}" -gt "0" ] && [ "${PARENT_DISTANCE}" -eq "0" ] && [ "${CHILD_DISTANCE}" -lt "${DIST}" ]; then
      PARENT=${CANDIDATE}
      DIST=${CHILD_DISTANCE}
    fi
  done <<< "$BRANCHES"

  if [ "${PARENT}" = "${CURRENT}" ]; then
    ROOTS[${#ROOTS[*]}]=${CURRENT}
  else
    WITH_PARENT[${CURRENT}]=${PARENT}
  fi
done <<< "$BRANCHES"

echo "[$ME] You are about to add ${ADDITION} onto ${OLD_BASE}. This will affect:"
echo "[$ME] Roots: ${ROOTS[*]}"
echo "[$ME] Children: ${!WITH_PARENT[@]}"
echo "[$ME] Tags:"
echo "${TAGS}"
read -p "[$ME] Continue? [Y/n] " -n 1 -r
echo    # (optional) move to a new line
if [[ $REPLY =~ ^[Yy]$ ]]; then
  DESTRUCTIVE=()
  CLEANUP=()

  echoAndEval git fetch

  echoAndEval git checkout ${OLD_BASE}

  echoAndEval git cherry-pick -n ${ADDITION}

  echoAndEval git commit --amend -C HEAD

  if [ "${DRY_RUN}" = true ]; then
    NEW_BASE="ccccccc"
  else
    NEW_BASE=$(git rev-parse --short HEAD) # 288c354
  fi

  echo "$N[$ME] Replacing ${OLD_BASE} with ${NEW_BASE}."

  NEW_BRANCHES=()
  for ROOT in "${ROOTS[*]}"; do
    OLD_BRANCH=$(echo ${ROOT}| cut -d'/' -f 2)
    NEW_BRANCH=${OLD_BRANCH}_on_${NEW_BASE}

    echo "$N[$ME] Creating ${NEW_BRANCH} from ${OLD_BRANCH} on ${NEW_BASE}."

    echoAndEval git checkout -b ${NEW_BRANCH} ${ROOT}
    NEW_BRANCHES[${#NEW_BRANCHES[*]}]=${NEW_BRANCH}
    echoAndEval git rebase --committer-date-is-author-date --preserve-merges -X ours --onto ${NEW_BASE} ${OLD_BASE}

	DESTRUCTIVE+=("git push --force origin ${NEW_BRANCH}:${OLD_BRANCH}")
	CLEANUP+=("git branch -d ${NEW_BRANCH}")

    moveChildren ${ROOT} ${NEW_BASE} ${NEW_BRANCH}
  done

  while read -r TAG; do
    MESSAGE="$(git --no-pager log -1 ${TAG} --pretty=%B)"
    NEW_COMMIT="$(eval git --no-pager log --grep=\'${MESSAGE}\' --pretty=%h ${NEW_BRANCHES})"

    echoAndEval git tag -d "${TAG}"
    echoAndEval git tag "${TAG}" "${NEW_COMMIT}"

    DESTRUCTIVE+=("git push origin :refs/tags/${TAG}")
  done <<< "$TAGS"

  DESTRUCTIVE+=("git push --tags")

  echo "${N}WARNING: Changes are done locally. Execute the following to update origin."
  for CMD in "${DESTRUCTIVE[@]}"; do
    echo "${CMD}"
  done

  echo "${N}Remove local copy branches:"
  for CMD in "${CLEANUP[@]}"; do
    echo "${CMD}"
  done
fi