git-quickfix

#!/bin/bash

set -Eeuo pipefail

if [ $# -lt 1 ]; then
  cat <<-EOT
	Usage: git quickfix <commit>

	    This command tries to do a quick commit fixup, for which you would usually
	    use an interactive rebase with fixup commits. This command tries to do the
	    same, adding the currently staged changes to the given commit, but without
	    touching your working tree, thus possibly being a lot faster.

	    The drawback is, that this tool cannot handle any conflicts and will simply
	    fail if any are encountered.

	    Make sure that there are no merge commits in the range <commit>..HEAD, it
	    they would probably get messed up badly.

	    Good luck!
	EOT
  exit 1
fi

COMMIT=$(git rev-parse "$1")
shift

SIGN_FLAG=
if [[ $(git config --get --type bool commit.gpgSign) == true ]]; then
  SIGN_FLAG=-S
fi

redo_commit () {
  COMMIT=$1
  PREV=$2

  TREE=$(git write-tree)

  echo $(git show -s --format='%B' $COMMIT | env \
    GIT_AUTHOR_NAME="$(git show -s --format='%an' $COMMIT)" \
    GIT_AUTHOR_EMAIL="$(git show -s --format='%ae' $COMMIT)" \
    GIT_AUTHOR_DATE="$(git show -s --format='%ad' $COMMIT)" \
    git commit-tree $SIGN_FLAG -p $PREV $TREE)
}

# Write a tree with the staged changes before switching the index file...
FIXED_TREE=$(git write-tree)

GIT_INDEX_FILE=$(git rev-parse --git-dir)/quickfix-index
export GIT_INDEX_FILE
trap "rm $GIT_INDEX_FILE" EXIT

PREV=$(git rev-parse $COMMIT^)
git read-tree $COMMIT

# Try to fixup the commit...
git show -s --format='Fixing %t %s' $COMMIT
if ! git diff-tree -p HEAD $FIXED_TREE | git apply --cached; then
  echo Could not apply changes....
  echo Aborting...
  exit 1
fi
PREV="$(redo_commit $COMMIT $PREV)"

# Replay the remaining commits
for COMMIT in $(git rev-list --reverse "$COMMIT.."); do
  git show -s --format='Applying %t %s' $COMMIT
  if ! git diff-tree -p $COMMIT | git apply --cached; then
    echo Could not apply changes....
    echo Aborting...
    exit 1
  fi

  PREV="$(redo_commit $COMMIT $PREV)"
done

git update-ref HEAD $PREV

exit 0