Recognize --head as an alias for --HEAD

Homebrew currently recognizes "--head" during the install process, but
rather than actually fetch HEAD it just nags the user to use the
correct option.	Since we recognize the spelling anyway, this just
promotes the lowercase version to an official alias.

Closes Homebrew/homebrew#26555.

Library/Homebrew/cmd/install.rb

@@ -19,10 +19,6 @@ module Homebrew extend self
       end
     end
 
-    if ARGV.include? '--head'
-      raise "Specify `--HEAD` in uppercase to build from trunk."
-    end
-
     ARGV.named.each do |name|
       # if a formula has been tapped ignore the blacklisting
       if not File.file? HOMEBREW_REPOSITORY/"Library/Formula/#{name}.rb"

Library/Homebrew/extend/ARGV.rb

@@ -111,7 +111,7 @@ module HomebrewArgvExtension
   end
 
   def build_head?
-    include? '--HEAD'
+    include?('--HEAD') || include?('--head')
   end
 
   def build_devel?