download/git: ensure we always work in the expected repository

download/git: ensure we always work in the expected repository

git always look directories up until it finds a repository. In case
the git cache is broken, it may no longer be identified as a repository,
and git will look higher in the directories until it finds one.

In the default conditions, this would be Buildroot's own git tree
(because DL_DIR is a subdir of Buildroot), but in some situations may
very well be any repository the user has Buildroot in, like a
br2-external tree...

So, we force git to use our git cache and never look elsewhere, as
Suggested by Ricardo.

Use GIT_DIR, as it has been there for ages now, while --git-dir was
only introduced later (even if most distros ship an later version),
as suggested by Arnout.

Also fix the one call to git that was not using the wrapper.

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>