Use mirror from github to clone git-repo tool

The repo tool is normally cloned from google, but in China the url
is blocked. Use mirror repo from github by default and add command
line parameter to allow using a custom url.

Change-Id: I91541fb1e42f8c668f7ff97d29232ca61eb3e20e
Reviewed-by: Karim Pinter <karim.pinter@theqtcompany.com>
Reviewed-by: aavit <eirik.aavitsland@theqtcompany.com>