pull: Support full recursive mirrors of repositories with summary file

Now that we have a summary file, we can use it to allow a simple:

ostree pull --mirror

To download the latest commit on every branch.  Also, for a case I'm
dealing with there's only one branch, but I don't want mirror users to
have to hardcode it.

https://bugzilla.gnome.org/show_bug.cgi?id=737807