Set committer date so older years appear on GitHub

[muhdbasiirzulkifle]

Summary

• GitHub contribution graphs use committer date, not author date. git commit --date only sets the author date, so generated history from past years was attributed to today.
• Set GIT_AUTHOR_DATE and GIT_COMMITTER_DATE when creating commits so older years (e.g. 2022-2025) show up on the profile.
• Document this in the README, including a git filter-branch rewrite for people who already generated history.

Test plan

• Run npx fake-git-history --startDate "2022/01/01" (or node src/cli.js locally).
• In the generated my-history repo, confirm author and committer years match:

  git log --format='%ad %cd' --date=format:'%Y' | awk '{print $1, $2}' | sort | uniq -c
  

• Push to a GitHub repo and check older years on the profile (year list on the left), not only the last 12 months.