git-submodule.sh: setup uninitialized variables

We have an environment variable `jobs=16` defined in our CI system, and this environment makes our build job failed with the following message: error: pathspec '16' did not match any file(s) known to git The pathspec '16' for Git command is from the environment variable "jobs". This is because "git-submodule" command is implemented in shell script, and environment variables may change its behavior. Set values for uninitialized variables, such as "jobs" and "recommend_shallow" will fix this issue. Helped-by: Jiang Xin <worldhello.net@gmail.com> Signed-off-by: Li Xuejiang <xuejiang@alibaba-inc.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>