Stop ignoring app/code/local/Mage

with 'mage' on the gitignore file, a user can't
add files customized from Mage

Magento.gitignore

@@ -89,7 +89,7 @@ LICENSE_AFL.txt
 LICENSE.html
 LICENSE.txt
 LICENSE_EE*
-mage
+/mage
 media/customer/
 media/dhl/
 media/downloadable/