CoreTap#install: fix clone target setting

When invoking `brew tap homebrew/core`, the input options will be like
`{ :clone_target => nil }`. This renders `Hash#merge` a dead operation.

It doesn't impact Homebrew itself, but could cause problem for
downstream projects Linxubrew/Tigerbrew when they adopt core/formula
separation.

Library/Homebrew/tap.rb

@@ -470,7 +470,7 @@ class CoreTap < Tap
   end
 
   def install(options = {})
-    options = { :clone_target => OFFICIAL_REMOTE }.merge(options)
+    options[:clone_target] ||= OFFICIAL_REMOTE
     super options
   end