download/git: ensure we have a sane repository

download/git: ensure we have a sane repository

There are cases where a repository might be broken, e.g. when a previous
operation was killed or otherwise failed unexpectedly.

We fix that by always initialising the repository, as suggested by
Ricardo. git-init is safe on an otherwise-healthy repository:

    Running git init in an existing repository is safe. It will not
    overwrite things that are already there. [...]

Using git-init will just ensure that we have the strictly required files
to form a sane tree. Any blob that is still missing would get fetched
later on.

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reported-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Tested-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>