Add a method for getting the repo HEAD

Library/Homebrew/cmd/config.rb

@@ -44,10 +44,7 @@ module Homebrew
   end
 
   def head
-    head = HOMEBREW_REPOSITORY.cd do
+    Homebrew.git_head || "(none)"
-      `git rev-parse --verify -q HEAD 2>/dev/null`.chomp
-    end
-    if head.empty? then "(none)" else head end
   end
 
   def origin

Library/Homebrew/tab.rb

@@ -14,10 +14,6 @@ class Tab < OpenStruct
     build = f.build.dup
     build.args = args
 
-    sha = HOMEBREW_REPOSITORY.cd do
-      `git rev-parse --verify -q HEAD 2>/dev/null`.chuzzle
-    end
-
     Tab.new :used_options => build.used_options,
             :unused_options => build.unused_options,
             :tabfile => f.prefix.join(FILENAME),
@@ -25,7 +21,7 @@ class Tab < OpenStruct
             :poured_from_bottle => false,
             :tapped_from => f.tap,
             :time => Time.now.to_i,
-            :HEAD => sha,
+            :HEAD => Homebrew.git_head,
             :compiler => compiler,
             :stdlib => stdlib
   end

Library/Homebrew/utils.rb

@@ -112,6 +112,10 @@ module Homebrew
     Process.wait(pid)
     $?.success?
   end
+
+  def self.git_head
+    HOMEBREW_REPOSITORY.cd { `git rev-parse --verify -q HEAD 2>/dev/null`.chuzzle }
+  end
 end
 
 def with_system_path