Copy this into .git/hooks/pre-commit (remove .sample)

#!/usr/bin/env ruby

require 'english'
require 'rubocop'

ADDED_OR_MODIFIED = /A|AM|^M/.freeze

changed_files = `git status --porcelain`.split(/\n/).
    select { |file_name_with_status|
      file_name_with_status =~ ADDED_OR_MODIFIED
    }.
    map { |file_name_with_status|
      file_name_with_status.split(' ')[1]
    }.
    select { |file_name|
      File.extname(file_name) == '.rb'
    }.join(' ')

system("rubocop #{changed_files}") unless changed_files.empty?

exit $CHILD_STATUS.exitstatus

Make it executable

$ chmod +x .git/hooks/pre-commit

You're good to go, from now it'd run rubocop against your changes and prevents from commiting unless you fix the offences.

How to skip pre-commit? just pass -n to git commit

$ git commit -n -m "[hotfix] blah blah"