add mirror script

this is a polling-based mirror script.

notable feature: does *not* use git clone --mirror, as object-by-object
clones of gerrit repositories are incredibly inefficient with normal git
clients.

Change-Id: Ic87381c97211dd2d5421d561a5b636f2191d7f3f
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Christian Stenger <christian.stenger@digia.com>