Don't add bottom 'match' line for deleted files

If a file is deleted, its new_pos is 0 (less than
total_blob_lines), but there is no reason to add the
bottom 'match' line in this case because there is not
extra content which could be expanded.

changelogs/unreleased/jprovazn-extra-line.yml

+---
+title: Don't show context button for diffs of deleted files.
+merge_request:
+author:
+type: fixed

lib/gitlab/diff/file.rb

@@ -250,7 +250,7 @@ module Gitlab
 
         last_line = lines.last
 
-        if last_line.new_pos < total_blob_lines(blob)
+        if last_line.new_pos < total_blob_lines(blob) && !deleted_file?
           match_line = Gitlab::Diff::Line.new("", 'match', nil, last_line.old_pos, last_line.new_pos)
           lines.push(match_line)
         end

spec/lib/gitlab/diff/file_spec.rb

@@ -26,6 +26,21 @@ describe Gitlab::Diff::File do
     end
   end
 
+  describe '#diff_lines_for_serializer' do
+    it 'includes bottom match line if not in the end' do
+      expect(diff_file.diff_lines_for_serializer.last.type).to eq('match')
+    end
+
+    context 'when deleted' do
+      let(:commit) { project.commit('d59c60028b053793cecfb4022de34602e1a9218e') }
+      let(:diff_file) { commit.diffs.diff_file_with_old_path('files/js/commit.js.coffee') }
+
+      it 'does not include bottom match line' do
+        expect(diff_file.diff_lines_for_serializer.last.type).not_to eq('match')
+      end
+    end
+  end
+
   describe '#mode_changed?' do
     it { expect(diff_file.mode_changed?).to be_falsey }
   end