This silents the following warning:

  Cloning into './debootstrap.git'...
  warning: redirecting to https://salsa.debian.org/installer-team/debootstrap.git/

See https://lists.debian.org/debian-devel-announce/2018/01/msg00004.html



Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>