download/git: add warning not to use our git cache

download/git: add warning not to use our git cache

We really want the user not to use our git cache manually, or their
changes (committed or not) may eventually get lost.

So, add a warning file, not unlike the one we put in the target/
directory, to warn the user not to use the git tree.

Ideally, we would have carried this file in support/misc/, but the git
backend does not have access to it: the working directory is somewhere
unknown, and TOPDIR is not exported in the environment.

So, we have to carry it in-line in the backend instead.

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