submodule update: learn `--[no-]recommend-shallow` option

submodule update: learn `--[no-]recommend-shallow` option

Sometimes the history of a submodule is not considered important by
the projects upstream. To make it easier for downstream users, allow
a boolean field 'submodule.<name>.shallow' in .gitmodules, which can
be used to recommend whether upstream considers the history important.

This field is honored in the initial clone by default, it can be
ignored by giving the `--no-recommend-shallow` option.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>