download/git: force gzip compression level 6

download/git: force gzip compression level 6

Force gzip compression level 6 when calculating hash of a downloaded GIT repo.
To make sure the tar->gzip->checksum chain always provides consistent result.`

The script was relying on the default compression level, which must not be
necessarily consistent among different gzip versions. The level 6 is gzip's
current default compression level.

Signed-off-by: Petr Kulhavy <brain@jikos.cz>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>