submodule: allow runtime enabling extensions.submodulePathConfig

submodule: allow runtime enabling extensions.submodulePathConfig

Add a new config `init.defaultSubmodulePathConfig` which allows
enabling `extensions.submodulePathConfig` for new submodules by
default (those created via git init or clone).

Important: setting init.defaultSubmodulePathConfig = true does
not globally enable `extensions.submodulePathConfig`. Existing
repositories will still have the extension disabled and will
require migration (for example via git submodule--helper command
added in the next commit).

Suggested-by: Patrick Steinhardt <ps@pks.im>
Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>