update-report: also create Cellar on migration.

Just on the off chance it doesn't exist (i.e. no-one has installed
anything yet or you're a `test-bot` that's run `git clean -xdf`
recently)

Library/Homebrew/cmd/update-report.rb

@@ -250,7 +250,7 @@ module Homebrew
       EOS
     end
 
-    Keg::ALL_TOP_LEVEL_DIRECTORIES.each do |dir|
+    (Keg::ALL_TOP_LEVEL_DIRECTORIES + ["Cellar"]).each do |dir|
       FileUtils.mkdir_p "#{HOMEBREW_PREFIX}/#{dir}"
     end