git-submodule.sh: setup uninitialized variables

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>