Added Danger file to lint commit messages

This Dangerfile ensures that commit authors write good commit messages.
We also prevent the use of Emoji, since these add no value and may not
always be displayed properly.

Merge commits are currently ignored. In the future we most likely want
to stop using these, but this is a separate discussion, hence we ignore
them for the time being.

The rules enforced by this Dangerfile are based on the article found at
https://chris.beams.io/posts/git-commit/. For more information, refer to
https://gitlab.com/gitlab-org/gitlab-ce/issues/50003.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/50003

Dangerfile

@@ -6,3 +6,4 @@ danger.import_dangerfile(path: 'danger/gemfile')
 danger.import_dangerfile(path: 'danger/database')
 danger.import_dangerfile(path: 'danger/documentation')
 danger.import_dangerfile(path: 'danger/frozen_string')
+danger.import_dangerfile(path: 'danger/commit_messages')

danger/commit_messages/Dangerfile

+# frozen_string_literal: true
+
+# rubocop: disable Style/SignalException
+# rubocop: disable Metrics/CyclomaticComplexity
+# rubocop: disable Metrics/PerceivedComplexity
+
+# Perform various checks against commits. We're not using
+# https://github.com/jonallured/danger-commit_lint because its output is not
+# very helpful, and it doesn't offer the means of ignoring merge commits.
+
+def fail_commit(commit, message)
+  fail("#{commit.sha}: #{message}")
+end
+
+def lines_changed_in_commit(commit)
+  commit.diff_parent.stats[:total][:lines]
+end
+
+def subject_starts_with_capital?(subject)
+  first_char = subject.chars.first
+
+  first_char.upcase == first_char
+end
+
+def ce_upstream?
+  gitlab.mr_labels.any? { |label| label == 'CE upstream' }
+end
+
+def lint_commits(commits)
+  failures = false
+
+  unicode_emoji_regex = %r((
+    [\u{1F300}-\u{1F5FF}] |
+    [\u{1F1E6}-\u{1F1FF}] |
+    [\u{2700}-\u{27BF}] |
+    [\u{1F900}-\u{1F9FF}] |
+    [\u{1F600}-\u{1F64F}] |
+    [\u{1F680}-\u{1F6FF}] |
+    [\u{2600}-\u{26FF}]
+  ))x
+
+  commits.each do |commit|
+    # For now we'll ignore merge commits, as getting rid of those is a problem
+    # separate from enforcing good commit messages.
+    next if commit.message.start_with?('Merge branch')
+
+    subject, separator, details = commit.message.split("\n", 3)
+
+    if subject.split.length < 3
+      fail_commit(
+        commit,
+        'The commit subject must contain at least three words'
+      )
+
+      failures = true
+    end
+
+    if subject.length > 50
+      fail_commit(
+        commit,
+        'The commit subject may not be longer than 50 characters'
+      )
+
+      failures = true
+    end
+
+    unless subject_starts_with_capital?(subject)
+      fail_commit(commit, 'The commit subject must start with a capital letter')
+      failures = true
+    end
+
+    if subject.end_with?('.')
+      fail_commit(commit, 'The commit subject must not end with a period')
+      failures = true
+    end
+
+    if separator && !separator.empty?
+      fail_commit(
+        commit,
+        'The commit subject and body must be separated by a blank line'
+      )
+
+      failures = true
+    end
+
+    details&.each_line do |line|
+      line = line.strip
+
+      next if line.length <= 72
+
+      url_size = line.scan(%r((https?://\S+))).sum { |(url)| url.length }
+
+      # If the line includes a URL, we'll allow it to exceed 72 characters, but
+      # only if the line _without_ the URL does not exceed this limit.
+      next if line.length - url_size <= 72
+
+      fail_commit(
+        commit,
+        'The commit body should not contain more than 72 characters per line'
+      )
+
+      failures = true
+    end
+
+    if !details && lines_changed_in_commit(commit) >= 20
+      fail_commit(
+        commit,
+        'Commits that change more than 20 lines ' \
+          'must describe these changes in the commit body'
+      )
+
+      failures = true
+    end
+
+    if commit.message.match?(/:[\+a-z0-9_\-]+:/)
+      fail_commit(
+        commit,
+        'Avoid the use of Markdown Emoji such as `:+1:`. ' \
+          'These add no value to the commit message, ' \
+          'and are displayed as plain text outside of GitLab'
+      )
+
+      failures = true
+    end
+
+    if commit.message.match?(unicode_emoji_regex)
+      fail_commit(
+        commit,
+        'Avoid the use of Unicode Emoji. ' \
+          'These add no value to the commit message, ' \
+          'and may not be displayed properly everywhere'
+      )
+
+      failures = true
+    end
+
+    if commit.message.match?(%r(([\w\-\/]+)?(#|!|&|%)\d+))
+      fail_commit(
+        commit,
+        'Use full URLs instead of short references ' \
+          '(`gitlab-org/gitlab-ce#123` or `!123`), as short references are ' \
+          'displayed as plain text outside of GitLab'
+      )
+
+      failures = true
+    end
+  end
+
+  if failures
+    markdown(<<~MARKDOWN)
+      ## Commit message standards
+
+      One or more commit messages do not meet our Git commit message standards.
+      For more information on how to write a good commit message, take a look at
+      [How to Write a Git Commit Message](https://chris.beams.io/posts/git-commit/).
+
+      Here is an example of a good commit message:
+
+          Reject ruby interpolation in externalized strings
+
+          When using ruby interpolation in externalized strings, they can't be
+          detected. Which means they will never be presented to be translated.
+
+          To mix variables into translations we need to use `sprintf`
+          instead.
+
+          Instead of:
+
+              _("Hello \#{subject}")
+
+          Use:
+
+              _("Hello %{subject}) % { subject: 'world' }
+
+      This is an example of a bad commit message:
+
+          updated README.md
+
+      This commit message is bad because although it tells us that README.md is
+      updated, it doesn't tell us why or how it was updated.
+    MARKDOWN
+  end
+end
+
+if git.commits.length > 10 && !ce_upstream?
+  warn(
+    'This merge request includes more than 10 commits. ' \
+      'Please rebase these commits into a smaller number of commits.'
+  )
+else
+  lint_commits(git.commits)
+end