Add WIP logic to linter

Lint the MR title without WIP
Apply suggestion to danger/commit_messages/Dangerfile
Delete prefix from subject if subject is a WIP
Add specs to commit_linter_spec.rb covering WIP

lib/gitlab/danger/commit_linter.rb

@@ -14,6 +14,7 @@ module Gitlab
       MAX_CHANGED_LINES_IN_COMMIT = 30
       SHORT_REFERENCE_REGEX = %r{([\w\-\/]+)?(#|!|&|%)\d+\b}.freeze
       DEFAULT_SUBJECT_DESCRIPTION = 'commit subject'
+      WIP_PREFIX = 'WIP: '
       PROBLEMS = {
         subject_too_short: "The %s must contain at least #{MIN_SUBJECT_WORDS_COUNT} words",
         subject_too_long: "The %s may not be longer than #{MAX_LINE_LENGTH} characters",
@@ -164,7 +165,7 @@ module Gitlab
       end
 
       def subject
-        message_parts[0]
+        message_parts[0].delete_prefix(WIP_PREFIX)
       end
 
       def separator

spec/lib/gitlab/danger/commit_linter_spec.rb

@@ -152,6 +152,18 @@ describe Gitlab::Danger::CommitLinter do
         end
       end
 
+      context 'when subject is a WIP' do
+        let(:final_message) { 'A B C' }
+        # commit message with prefix will be over max length. commit message without prefix will be of maximum size
+        let(:commit_message) { described_class::WIP_PREFIX + final_message + 'D' * (described_class::WARN_SUBJECT_LENGTH - final_message.size) }
+
+        it 'does not have any problems' do
+          commit_linter.lint
+
+          expect(commit_linter.problems).to be_empty
+        end
+      end
+
       context 'when subject is too short and too long' do
         let(:commit_message) { 'A ' + 'B' * described_class::MAX_LINE_LENGTH }
 
@@ -183,7 +195,7 @@ describe Gitlab::Danger::CommitLinter do
         end
       end
 
-      context 'when subject ands with a period' do
+      context 'when subject ends with a period' do
         let(:commit_message) { 'A B C.' }
 
         it 'adds a problem' do