Add repo init --depth parameter

Using e.g.

  repo init --depth 1

saves a lot of bandwidth and disk space if you do not
need the git history. So allow the user to specify
the checkout depth by the REPO_INIT_DEPTH environment
variable. By default the full history is fetched.

Signed-off-by: Micha M <contact-micha+efoundation@posteo.de>