Add --git option to 'brew install -i'.

This allows for easy generation of diffs, per a trick in
http://wiki.github.com/mxcl/homebrew/formula-cookbook

Library/Homebrew/install.rb

@@ -78,6 +78,15 @@ def install f
         ohai "Entering interactive mode"
         puts "Type `exit' to return and finalize the installation"
         puts "Install to this prefix: #{f.prefix}"
+
+        if ARGV.flag? '--git'
+          system "git init"
+          system "git add -A"
+          puts "This folder is now a git repo. Make your changes and then use:"
+          puts "  git diff | pbcopy"
+          puts "to copy the diff to the clipboard."
+        end
+
         interactive_shell
         nil
       elsif ARGV.include? '--help'