submodule-config: keep shallow recommendation around

submodule-config: keep shallow recommendation around

The shallow field will be used in a later patch by `submodule update`.
To differentiate between the actual depth (which may be different),
we name it `recommend_shallow` as the field in the .gitmodules file
is only a recommendation by the project.

Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>