PreCommit:
  RuboCop:
    enabled: true
    problem_on_unmodified_line: ignore
    flags: ['--fail-level', 'error']

PreCommit:
  RuboCop:
    enabled: true
    problem_on_unmodified_line: ignore
    flags: ['--fail-level', 'error', '--format', 'emacs']

Set the RuboCop output format to emacs
Set the RuboCop output format to emacs

This produces output compatible with overcommit.
