Refactor commit_messages#lint_commit

This introduces additional method for linting,
to reduce the complexity of `lint_commits`.

danger/commit_messages/Dangerfile

@@ -64,11 +64,12 @@ def too_many_changed_lines?(commit)
     lines_changed_in_commit(commit) >= 30
 end
 
-def lint_commits(commits)
-  failures = false
-  emoji_checker = EmojiChecker.new
+def emoji_checker
+  @emoji_checker ||= EmojiChecker.new
+end
 
-  unicode_emoji_regex = %r((
+def unicode_emoji_regex
+  @unicode_emoji_regex ||= %r((
     [\u{1F300}-\u{1F5FF}] |
     [\u{1F1E6}-\u{1F1FF}] |
     [\u{2700}-\u{27BF}] |
@@ -77,15 +78,17 @@ def lint_commits(commits)
     [\u{1F680}-\u{1F6FF}] |
     [\u{2600}-\u{26FF}]
   ))x
+end
 
-  commits.each do |commit|
+def lint_commit(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')
+  return false if commit.message.start_with?('Merge branch')
 
   # We ignore revert commits as they are well structured by Git already
-    next if commit.message.start_with?('Revert "')
+  return false if commit.message.start_with?('Revert "')
 
+  failures = false
   subject, separator, details = commit.message.split("\n", 3)
 
   if subject.split.length < 3
@@ -191,9 +194,16 @@ def lint_commits(commits)
 
     failures = true
   end
+
+  failures
+end
+
+def lint_commits(commits)
+  failed = commits.reject do |commit|
+    lint_commit(commit)
   end
 
-  if failures
+  if failed.any?
     markdown(<<~MARKDOWN)
       ## Commit message standards