submodule--helper: add is-active subcommand

submodule--helper: add is-active subcommand

The definition of which submodules are of interest by the user
is tied to the configuration submodule.<name>.url; when it is
set to a non-empty string, it is of interest.  We'd want to be
able to later change this definition, but there are many places
that explicitly check this condition in the scripted Porcelain.

Introduce the "is-active" subcommand to "submodule--helper", so
that the exact definition of what submodule is of interest can
be centrally defined (and changed in later steps).  In a few
patches that follow, this helper is used to replace the explicit
checks of the configuration variable in scripts.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>