download/git: fix transform regexp for older tar versions

Older versions of tar (e.g. 1.27.1) incorrectly interpret the escaping
of the regexp separator, and generate broken tarballs.

For example, given the following transform expression:
    --transform="s/^\.\//squashfs-e38956b92f738518c29734399629e7cdb33072d3\//"

the resulting paths in the generated tarball would be:
    squashfs-e38956b92f738518c29734399629e7cdb33072d3\/

i.e. a directory which last character is indeed a '\'.

We fix that by using a separator which is very unlikely to occur in a
filename.

Fixes:
    http://autobuild.buildroot.org/results/742/7427f34e5c9f6d043b0fe6ad2c66cc0f31d2b24f/



and probably a slew of others as well...

Take this opportunity to fix indentation on the following line
(leading spaces, not TABs).

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