Add a commit counter rule (errors out when more than 20 commits)

Context

Closes gitlab-org/gitlab#358125 (closed)

What does this MR do?

Add a rule for raising an error when we have more than 20 commits.

What does it look like?

When a branch has more than 20 commits ahead of master, the following error message will be displayed by Danger:

This MR has more than 20 commits. You need to rebase this branch to have fewer commits.

How to test locally?

• Go to your local GitLab installation
• git checkout -b branch_with_21_commits
• Change your Gemfile:

# Before
gem 'gitlab-dangerfiles', '~> 3.0', require: false

# After
gem 'gitlab-dangerfiles', git: 'https://gitlab.com/gitlab-org/ruby/gems/gitlab-dangerfiles', branch: '358125-no_more_than_20_commits'

• bundle
• Check: bundle info gitlab-dangerfiles should give you version 3.1.1
• We'll now create 21 commits on a new branch, and run danger locally:

# Should pass
bundle exec rake danger_local

# Create 21 commits
{
  touch test-file.out
  i=0
  while [ $i -ne 21 ]
  do
          i=$(($i+1))
          echo "$i" >> test-file.out
          git add .
          git commit -m "This is commit #${i}"
  done
}

# Should fail
bundle exec rake danger_local

• Remove local branch:

git checkout master
git branch -D branch_with_21_commits