Fix highlighting of deleted lines in diffs.

CHANGELOG

@@ -40,6 +40,7 @@ v 8.0.0 (unreleased)
   - Add ability to get user information by ID of an SSH key via the API
   - Fix bug which IE cannot show image at markdown when the image is raw file	of gitlab  
   - Add support for Crowd
+  - Fix highlighting of deleted lines in diffs.
 
 v 7.14.1
   - Improve abuse reports management from admin area

config/initializers/rouge_diff_lexer.rb

+# Here until https://github.com/jneen/rouge/pull/297 is merged into Rouge and the gem is updated in GitLab.
+module Rouge
+  module Lexers
+    class Diff
+      def self.analyze_text(text)
+        return 1   if text.start_with?('Index: ')
+        return 1   if text.start_with?('diff ')
+        return 0.9 if text.start_with?('--- ')
+      end
+
+      state :root do
+        rule(/^ .*\n/, Text)
+        rule(/^---\n/, Text)
+        rule(/^\+.*\n/, Generic::Inserted)
+        rule(/^-+.*\n/, Generic::Deleted)
+        rule(/^!.*\n/, Generic::Strong)
+        rule(/^@.*\n/, Generic::Subheading)
+        rule(/^([Ii]ndex|diff).*\n/, Generic::Heading)
+        rule(/^=.*\n/, Generic::Heading)
+        rule(/.*\n/, Text)
+      end
+    end
+  end
+end