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