versions: restore original constant

Before:

  f1 = Formula.factory('tree')
  f1.versions
  f2 = Formula.factory('tree')
  f1.class == f2.class # => false

After:

  f1 = Formula.factory('tree')
  f1.versions
  f2 = Formula.factory('tree')
  f1.class == f2.class # => true

Library/Homebrew/cmd/versions.rb

@@ -108,7 +108,7 @@ class Formula
 
         # Unload the class so Formula#version returns the correct value
         begin
-          Formulary.unload_formula name
+          old_const = Formulary.unload_formula name
           nostdout { yield Formula.factory(path.to_s) }
         rescue *IGNORED_EXCEPTIONS => e
           # We rescue these so that we can skip bad versions and
@@ -116,6 +116,8 @@ class Formula
           ohai "#{e} in #{name} at revision #{sha}", e.backtrace if ARGV.debug?
         rescue FormulaUnavailableError
           # Suppress this error
+        ensure
+          Formulary.restore_formula name, old_const
         end
       end
     end

Library/Homebrew/formulary.rb

@@ -13,6 +13,13 @@ class Formulary
     Object.const_get(Formula.class_s(formula_name))
   end
 
+  def self.restore_formula formula_name, value
+    old_verbose, $VERBOSE = $VERBOSE, nil
+    Object.const_set(Formula.class_s(formula_name), value)
+  ensure
+    $VERBOSE = old_verbose
+  end
+
   # A FormulaLoader returns instances of formulae.
   # Subclasses implement loaders for particular sources of formulae.
   class FormulaLoader