The `after_save` callback was looking for changes only in the URL without credentials,
this was happening because we store the credentials in a separate field.

* Use Rugged filters to work with local/remote branches.
* Rename Project#remote_mirror? to Project#has_remote_mirror?
* Use delegate pattern for Repository#push_remote_branches and Repository#delete_remote_branches
* Make Repository#remote_tags return Gitlab::Git::Tag objects.
* Refactor Repository#remote_branches
* Rename Repository#push_branches to Repository#push_remote_branches
* Made other small refactors to views and specs.
* Make encrypted_credentials column size bigger.
  This was giving problems with MySql for large passwords.
* Refactor for UpdateRemoteMirrorService spec so we no longer
  harcode the branch names and specs are not broken after `gitlab-test`
  project is updated.

* Now #url= and #url always handle the full URL, encryption is done transparently
* #safe_url was added for future purposes
* Little refactor for UpdateRemoteMirrorService