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>
(cherry picked from commit 04a22cf1b521acb5634ed083e0381d42979d1698)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/download/git

@@ -98,4 +98,4 @@ LC_ALL=C sort <"${basename}.list" >"${basename}.list.sorted"
 # sources.buildroot.org and used in the *.hash files
 tar cf - --numeric-owner --owner=0 --group=0 --mtime="${date}" --format=gnu \
          -T "${basename}.list.sorted" >"${output}.tar"
-gzip -n <"${output}.tar" >"${output}"
+gzip -6 -n <"${output}.tar" >"${output}"