For each changed file, generate a separate "fixup" commit targetting the last time that file was changed.

git-flatfix

#!/usr/bin/env ruby

# git-flatfix
# For each changed file, generate a separate "fixup" commit targetting the last
# time that file was changed.

def usage
  STDERR.puts <<~USAGE
    Usage: git-flatfix [basebranch]
  USAGE
  exit 1
end

def all_branches
  @all_branches ||= `git branch --list --remotes --format='%(refname:lstrip=-2)'`.chomp.split("\n")
end

def detect_main_branch
  (%w[origin/main origin/trunk origin/master] & all_branches).first
end

base = ARGV[0] || detect_main_branch
usage unless base || base == "--help" || ARGV.size > 1

changes = `git status --porcelain`.lines.grep(/\A M/)

if changes.empty?
  STDERR.puts "No changes"
  exit 0
end

changed_files = changes.map { |line| line[/\A M (.*)\n\z/m, 1] }

changed_files.map do |f|
  last_change = `git log -n 1 --pretty=format:%H #{base}...HEAD -- #{f}`.chomp
  if last_change.empty?
    warn "Could not find last change for #{f}"
  else
    cmd = "git commit --quiet --fixup=#{last_change} -- #{f}"
    puts cmd
    system(cmd)
  end
end