Merge branch 'bw/submodule-is-active'

Merge branch 'bw/submodule-is-active'

"what URL do we want to update this submodule?" and "are we
interested in this submodule?" are split into two distinct
concepts, and then the way used to express the latter got extended,
paving a way to make it easier to manage a project with many
submodules and make it possible to later extend use of multiple
worktrees for a project with submodules.

* bw/submodule-is-active:
  submodule add: respect submodule.active and submodule.<name>.active
  submodule--helper init: set submodule.<name>.active
  clone: teach --recurse-submodules to optionally take a pathspec
  submodule init: initialize active submodules
  submodule: decouple url and submodule interest
  submodule--helper clone: check for configured submodules using helper
  submodule sync: use submodule--helper is-active
  submodule sync: skip work for inactive submodules
  submodule status: use submodule--helper is-active
  submodule--helper: add is-active subcommand