Cache outdated_versions for Formula

Closes #584.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/formula.rb

@@ -1001,10 +1001,15 @@ class Formula
 
   # @private
   def outdated_versions(options = {})
-    @outdated_versions ||= begin
+    @outdated_versions ||= Hash.new do |cache, key|
-      all_versions = []
-
       raise Migrator::MigrationNeededError.new(self) if migration_needed?
+      cache[key] = _outdated_versions(key)
+    end
+    @outdated_versions[options]
+  end
+
+  def _outdated_versions(options = {})
+    all_versions = []
 
     installed_kegs.each do |keg|
       version = keg.version
@@ -1014,11 +1019,10 @@ class Formula
     end
 
     head_version = latest_head_version
-      if head_version
+    if head_version && !head_version_outdated?(head_version, options)
-        head_version_outdated?(head_version, options) ? all_versions.sort! : []
+      []
     else
-        all_versions.sort!
+      all_versions.sort
-      end
     end
   end
 

Library/Homebrew/test/test_formula.rb

@@ -583,6 +583,7 @@ class OutdatedVersionsTests < Homebrew::TestCase
 
   def reset_outdated_versions
     f.instance_variable_set(:@outdated_versions, nil)
+    f.instance_variable_set(:@outdated_versions_head_fetched, nil)
   end
 
   def test_greater_different_tap_installed