Incorrect URL validation "remote mirrors url"

Summary

Unable to update "Push to a remote repository" => "Git repository URL" to same import url/"Pull from a remote repository" => "Git repository URL"

Steps to reproduce

  1. Import/mirror a repository (by url) - https://gitlab.com/projects/new
  2. Navigate to the mirror settings https://gitlab.com/<user>/<project>/mirror
  3. Update the "Push to a remote repository" => "Git repository URL" to the same value as the "Pull from a remote repository" => "Git repository URL"

See: https://drive.google.com/file/d/0B_4wYK1qcPT1RXFockQzd19xaGs/view?usp=sharing

Expected behavior

Settings successfully updated.

Actual behavior

The form contains the following error: Remote mirrors url is already in use - https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/models/remote_mirror.rb#L130-134

//cc @jameslopez @rdavila