title: Extra changeColumns in Doctrine generate-migrations-diff
body: <p>I'm generating migrations between different yaml schema files: i.e. running:</p>
  
  <p>symfony doctrine:generate-migrations-diff</p>
  
  <p>And the resulting migration file has a whole slew of changeColumn calls that weren't added in the last schema file change.</p>
  
  <p>For example, if you run generate-migrations-diff without changing your schema file whatsoever, you should get an empty up() function.  However, the function that results for me has a changeColumn call for virtually every table in my database.</p>
  
  <p>Am i doing something wrong or is this a bug?</p>