clone: add failing test for a mirror-clone with clone_into

Show a failure to perform a mirror-clone from a repository, both local
and remote.
clone: duplicate the remote

Instead of changing the user-provided remote, duplicate it so we can add
the extra refspec without having to worry about unsetting it before
returning.
clone: don't error out if the branch already exists

We set up the current branch after we fetch from the remote. This means
that the user's refspec may have already created this reference. It is
therefore not an error if we cannot create the branch because it already
exists.

This allows for the user to replicate git-clone's --mirror option.
Merge pull request #2354 from libgit2/cmn/clone-into-mirror

Allow mirror-clone via `git_clone_into()`