Quibble clones each of the repositories serially. It does so by invoking zuul-cloner which has a very lame foreach implementation for cloning.
Potentially we could use a threadpool or use git submodules and delegate parallelization to it.
GOTCHA: there might be name clash when cloning. Eg if one clones mediawiki/extensions/Foo first, that creates the directory ./extensions and then prevent us from cloning mediawiki/core in it (iirc). So order of cloning does matter.