Store the formula used to build the keg in the keg

Store the formula used to build the keg inside the keg in a
file named NAME/VERSION/.brew/NAME.rb after removing the
bottle do ... end block.

See https://github.com/Homebrew/brew-evolution/pull/6.
Closes https://github.com/Homebrew/brew/issues/931.

Library/Homebrew/cmd/list.rb

@@ -140,6 +140,8 @@ class PrettyListing
           # dylibs have multiple symlinks and we don't care about them
           (pnn.extname == ".dylib" || pnn.extname == ".pc") && !pnn.symlink?
         end
+      when ".brew"
+        # Ignore .brew
       else
         if pn.directory?
           if pn.symlink?

Library/Homebrew/formula_installer.rb

@@ -260,6 +260,12 @@ class FormulaInstaller
       compute_and_install_dependencies if not_pouring && !ignore_deps?
       build
       clean
+
+      # Store the formula used to build the keg in the keg.
+      s = formula.path.read.gsub(/  bottle do.+?end\n\n?/m, "")
+      brew_prefix = formula.prefix/".brew"
+      brew_prefix.mkdir
+      Pathname(brew_prefix/"#{formula.name}.rb").atomic_write(s)
     end
 
     build_bottle_postinstall if build_bottle?

Library/Homebrew/test/test_formula_installer.rb

@@ -58,6 +58,7 @@ class InstallTests < Homebrew::TestCase
       bin = HOMEBREW_PREFIX+"bin"
       assert_predicate bin, :directory?
       assert_equal 3, bin.children.length
+      assert_predicate f.prefix/".brew/testball.rb", :readable?
     end
   end