When a Gemfile installs rails master, it clones rails for
12 times. By storing Rails‘s subgems in the pending queue under the same name "rails". We can avoid clone rails gems
multiple times.

Use a hash to keep track of whether we've already checked out a repo by remote+revision.

.. and then show them all in the "Using" message, too.

When there is no lockfile, we've already cloned the repository earlier to resolve the lockfile. So the check here will return false and some git gems message will not appear.