Add ARGV.build_head? and use it.

This allows both "--HEAD" and "-H" to be used consistently, which was the
intention in the first place.

Library/Homebrew/extend/ARGV.rb

@@ -51,6 +51,9 @@ module HomebrewArgvExtension
   def interactive?
     flag? '--interactive'
   end
+  def build_head?
+    flag? '--HEAD'
+  end
 
   def flag? flag
     options.each do |arg|