Enable Rubocop Performance/ReverseEach

`Array.reverse_each` is faster than `Array.reverse.each` because:

* reverse.each creates a new array then loops each element
* reverse_each loops in reverse order (no intermediate array created)

.rubocop_todo.yml

@@ -262,14 +262,6 @@ Naming/HeredocDelimiterNaming:
 Naming/RescuedExceptionsVariableName:
   Enabled: false
-# Offense count: 3
-# Cop supports --auto-correct.
-Performance/ReverseEach:
-  Exclude:
-    - 'app/models/commit.rb'
-    - 'db/migrate/20190222051615_add_indexes_for_merge_request_diffs_query.rb'
-    - 'lib/gitlab/profiler.rb'
-
 # Offense count: 7081
 # Configuration parameters: Prefixes.
 # Prefixes: when, with, without

app/models/commit.rb

@@ -346,7 +346,7 @@ class Commit
       if commits_in_merge_request.present?
         message_body << ""
-        commits_in_merge_request.reverse.each do |commit_in_merge|
+        commits_in_merge_request.reverse_each do |commit_in_merge|
           message_body << "#{commit_in_merge.short_id} #{commit_in_merge.title}"
         end
       end

db/migrate/20190222051615_add_indexes_for_merge_request_diffs_query.rb

@@ -35,7 +35,7 @@ class AddIndexesForMergeRequestDiffsQuery < ActiveRecord::Migration[5.0]
   end
   def down
-    INDEX_SPECS.reverse.each do |spec|
+    INDEX_SPECS.reverse_each do |spec|
       remove_concurrent_index(*spec)
     end
   end

lib/gitlab/profiler.rb

@@ -166,7 +166,7 @@ module Gitlab
         [model, times.count, times.sum]
       end
-      summarised_load_times.sort_by(&:last).reverse.each do |(model, query_count, time)|
+      summarised_load_times.sort_by(&:last).reverse_each do |(model, query_count, time)|
         logger.info("#{model} total (#{query_count}): #{time.round(2)}ms")
       end
     end