git-afix is a smart fixup tool that creates fixup commit with each staged file. Fixed revision is last rev that the file was changed in.

git-afix

#!/bin/bash
set -euo pipefail
IFS=$'\n\t'

fixups=""
for FILE in $(git --no-pager diff --name-only --cached); do
    # TODO limit to last merge commit
    REF=$(git log -n 1 --follow --no-merges --pretty="format:%H" -- "$FILE")
    fixups+="$REF $FILE"$'\n'
done

# consolidate fixups of same ref to one fixup commit
commits=$(echo "$fixups" | sort | sed 1d | awk 'BEGIN {ORS=""} {if (LAST!=$1) print "\n"$1; print " "$2; LAST=$1}' | sed 1d)

while IFS= read -r LINE; do
    REF=$(echo "$LINE" | awk '{print $1}')
    FILES=$(echo "$LINE" | awk '{$1=""; print $0}')

    echo "$FILES" | xargs git commit --fixup="$REF" --
done <<< "$commits"