Use tag_exists? in GitAccess#protected_tag?

This removes the need for retrieving the entire list of tags just to
check if a specific one exists.

lib/gitlab/git_access.rb

@@ -176,7 +176,7 @@ def protected_branch_action(oldrev, newrev, branch_name)
     end
 
     def protected_tag?(tag_name)
-      project.repository.tag_names.include?(tag_name)
+      project.repository.tag_exists?(tag_name)
     end
 
     def user_allowed?