Filters out duplicate offences for Github formatters

When the same exact message is repeated on the same line,
this change removes the duplicates.  The test was updated
to reflect this changed behavior as well.

lib/pronto/formatter/github_formatter.rb

@@ -2,6 +2,8 @@ module Pronto
   module Formatter
     class GithubFormatter
       def format(messages, repo)
+        messages = messages.uniq { |message| [message.msg, message.line.new_lineno] }
+
         commit_messages = messages.map do |message|
           sha = message.commit_sha
           body = message.msg

lib/pronto/formatter/github_pull_request_formatter.rb

@@ -2,6 +2,8 @@ module Pronto
   module Formatter
     class GithubPullRequestFormatter
       def format(messages, repo)
+        messages = messages.uniq { |message| [message.msg, message.line.new_lineno] }
+
         commit_messages = messages.map do |message|
           body = message.msg
           path = message.path

spec/pronto/formatter/github_formattter_spec.rb

@@ -13,6 +13,33 @@ module Formatter
         let(:line) { double(new_lineno: 1, commit_sha: '123', position: nil) }
         before { line.stub(:commit_line).and_return(line) }
 
+        specify do
+          Octokit::Client.any_instance
+                         .should_receive(:commit_comments)
+                         .once
+                         .and_return([])
+
+          Octokit::Client.any_instance
+                         .should_receive(:create_commit_comment)
+                         .once
+
+          subject
+        end
+      end
+
+      describe '#format without duplicates' do
+        subject { github_formatter.format(messages, repository) }
+        let(:messages) { [message1, message2] }
+        let(:repository) { Git::Repository.new('.') }
+        let(:message1) { Message.new('path/to1', line1, :warning, 'crucial') }
+        let(:message2) { Message.new('path/to2', line2, :warning, 'crucial') }
+        let(:line1) { double(new_lineno: 1, commit_sha: '123', position: nil) }
+        let(:line2) { double(new_lineno: 2, commit_sha: '123', position: nil) }
+        before do
+          line1.stub(:commit_line).and_return(line1)
+          line2.stub(:commit_line).and_return(line2)
+        end
+
         specify do
           Octokit::Client.any_instance
                          .should_receive(:commit_comments)