stage_fixed (or equivalent) for changed files not originally staged

[isturdy]

:zap: Summary

IIUC, currently stage_fixed only affects the files originally in the glob. I'm trying to use lefthook for code/documentation generation, and so some steps create/modify files not part of the original staged set/glob results.

Value

Allows using pre-commit for generated files (e.g. documentation)

Behavior and configuration changes

For a silly example, consider a hook that duplicates files:

pre-commit:
  commands:
    clone:
      stage_fixed: true
      glob: "*.txt"
      run: cp {staged_files} backup

I initially expected that this would commit both the manually-created file and its backup, but instead it commits the original file and creates the backup but does not stage/commit it. I would like to see a variant of stage_fixed that also adds any files not originally staged for the commit that change during the command.