Improve indexes on merge_request_diffs

For getting the SHAs from an MR to find pipelines, we get the last 100 MR diffs
for the MR, and find commits from those. This was un-indexed before, because the
index was not a composite index on merge_request_diff_id, id. Changing that
means that this scope can exclusively use indexes.

db/post_migrate/20171124095655_add_index_on_merge_request_diffs_merge_request_id_and_id.rb

+class AddIndexOnMergeRequestDiffsMergeRequestIdAndId < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index(:merge_request_diffs, [:merge_request_id, :id])
+  end
+
+  def down
+    if index_exists?(:merge_request_diffs, [:merge_request_id, :id])
+      remove_concurrent_index(:merge_request_diffs, [:merge_request_id, :id])
+    end
+  end
+end

db/post_migrate/20171124100152_remove_index_on_merge_request_diffs_merge_request_diff_id.rb

+class RemoveIndexOnMergeRequestDiffsMergeRequestDiffId < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    if index_exists?(:merge_request_diffs, :merge_request_id)
+      remove_concurrent_index(:merge_request_diffs, :merge_request_id)
+    end
+  end
+
+  def down
+    add_concurrent_index(:merge_request_diffs, :merge_request_id)
+  end
+end

db/schema.rb

@@ -1020,7 +1020,7 @@ ActiveRecord::Schema.define(version: 20171124132536) do
     t.string "start_commit_sha"
   end
 
-  add_index "merge_request_diffs", ["merge_request_id"], name: "index_merge_request_diffs_on_merge_request_id", using: :btree
+  add_index "merge_request_diffs", ["merge_request_id", "id"], name: "index_merge_request_diffs_on_merge_request_id_and_id", using: :btree
 
   create_table "merge_request_metrics", force: :cascade do |t|
     t.integer "merge_request_id", null: false