Merge branch 'ab/submodule-helper-prep-only'

Merge branch 'ab/submodule-helper-prep-only'

Preparation to remove git-submodule.sh and replace it with a builtin.

* ab/submodule-helper-prep-only:
  submodule--helper: use OPT_SUBCOMMAND() API
  submodule--helper: drop "update --prefix <pfx>" for "-C <pfx> update"
  submodule--helper: remove --prefix from "absorbgitdirs"
  submodule API & "absorbgitdirs": remove "----recursive" option
  submodule.c: refactor recursive block out of absorb function
  submodule tests: test for a "foreach" blind-spot
  submodule--helper: fix a memory leak in "status"
  submodule tests: add tests for top-level flag output
  submodule--helper: move "config" to a test-tool