submodule--helper: add gitdir migration command

submodule--helper: add gitdir migration command

Manually running
"git config submodule.<name>.gitdir .git/modules/<name>"
for each submodule can be impractical, so add a migration command to
submodule--helper to automatically create configs for all submodules
as required by extensions.submodulePathConfig.

The command calls create_default_gitdir_config() which validates the
gitdir paths before adding the configs.

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