RC TEST: test basics

TODO: divide (how?)
Revert upgrade worktrees

Reverts: d405ddc6a4 eb6330f4e8
path: implement common_dir handling in git_path_submodule()

This allows making submodules a linked workdirs.

Same as for .git, but ignores the GIT_COMMON_DIR environment variable,
because it would mean common directory for the parent repository and
does not make sense for submodule.
RC TEST: for inquire

TODO: probably setup of them can be used without inquire
t7410-submodule-checkout-to.sh: more cases for "update --init"

Verify that the resulted submodules are always independent on their behavior.
Do not check the internal data of repositories. So, any valid implementation
should pass the tests.
submodule: refactor to allow positional parameters
submodule refactor: use rev-path --git-path instead of manually constructed paths
submodule: make update --init create linked submodule worktree when called in linked main worktree
TEST: test after adding
IMPL: properly git submodue add
Merge branch 'export/tmp/multiplewt-vs-submodules' after rebase

Cleaned it up, to make later submission easier