scripts: check if .git exists before checking submodule status

scripts: check if .git exists before checking submodule status

Currently we check status of each submodule, before actually checking
if we're in a git repo. These status commands will all fail, but we
are hiding their output so we don't see it currently.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>