Relax Danger requirements about changed lines

This relaxes the commit rules regarding the number of changed lines.
Previously we required a detailed description if 20 or more lines were
changed. Starting with this commit we change the rules so Danger only
notifies you about writing a detailed description if:

1. A commit changes more than 3 files
2. The same commit changes more than 30 lines

This should make it easier to do a find replace to fix a typo, without
having to repeat what the subject line already says, while still
reminding developers to write a detailed description for new features
and other larger changes.

Fixes https://gitlab.com/gitlab-org/release/framework/issues/13

danger/commit_messages/Dangerfile

@@ -26,6 +26,11 @@ def ce_upstream?
   gitlab.mr_labels.any? { |label| label == 'CE upstream' }
 end
 
+def too_many_changed_lines?(commit)
+  commit.diff_parent.stats[:total][:files] > 3 &&
+    lines_changed_in_commit(commit) >= 30
+end
+
 def lint_commits(commits)
   failures = false
 
@@ -102,10 +107,10 @@ def lint_commits(commits)
       failures = true
     end
 
-    if !details && lines_changed_in_commit(commit) >= 20
+    if !details && too_many_changed_lines?(commit)
       fail_commit(
         commit,
-        'Commits that change more than 20 lines ' \
+        'Commits that change 30 or more lines in more than three files ' \
           'must describe these changes in the commit body'
       )