feat: pass numeric placeholders to files command

[fabn]

It address the use case outlined here

#315 (comment)

post-checkout:
  commands:
    01-bundle-install:
      files: git diff --name-only {1} {2}
      glob: '*Gemfile'
      run: bundle install

⚡ Summary

☑️ Checklist

• Check locally
• Add tests

I wasn't able to test it because the line I changed is not covered by current tests

[mrexox]

Thank you! 🚀