support/download: further silence the git helper

We expresely call printf in the git helper, calls which were not
addresed in the previous silent-build patchset.

Just redirect stdout to oblivion when being silent.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Tested-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>