Enable Rubocop Casecmp Performance Cop.

Also fixes the errors caused by enabling the cop. casecmp is more performant than `.downcase` and `==`.

.rubocop.yml

@@ -937,10 +937,9 @@ Lint/Void:
 
 ##################### Performance ############################
 
-# TODO: Enable Casecmp Cop.
 # Use `casecmp` rather than `downcase ==`.
 Performance/Casecmp:
-  Enabled: false
+  Enabled: true
 
 # TODO: Enable DoubleStartEndWith Cop.
 # Use `str.{start,end}_with?(x, ..., y, ...)` instead of

app/models/repository.rb

@@ -453,7 +453,7 @@ class Repository
   def version
     cache.fetch(:version) do
       tree(:head).blobs.find do |file|
-        file.name.downcase == 'version'
+        file.name.casecmp('version').zero?
       end
     end
   end

lib/gitlab/database.rb

@@ -5,11 +5,11 @@ module Gitlab
     end
 
     def self.mysql?
-      adapter_name.downcase == 'mysql2'
+      adapter_name.casecmp('mysql2').zero?
     end
 
     def self.postgresql?
-      adapter_name.downcase == 'postgresql'
+      adapter_name.casecmp('postgresql').zero?
     end
 
     def self.version

lib/gitlab/markup_helper.rb

@@ -40,7 +40,7 @@ module Gitlab
     # Returns boolean
     def plain?(filename)
       filename.downcase.end_with?('.txt') ||
-        filename.downcase == 'readme'
+        filename.casecmp('readme').zero?
     end
 
     def previewable?(filename)