Support to clone repositories in parallel
Quibble clones each of the repositories serially. It does so by invoking
zuul.Cloner() which has a very lame foreach implementation for cloning.
When only one worker is requested, use Cloner.execute() as before. This
way the new behavior is feature flagged.
Add a new option --git-parallel, defaulting to one worker.
Logging of the clone operations is multiplexed and hard to gasp:
- remove Zuul.Repo() DEBUG level, it added little value.
- each clone logging is now suffixed with the repository name
If mediawiki/core has not been cloned yet, clone it first which I think
might avoid errors due to git clone and paths conflicts.