submodule: stop sanitizing config options

submodule: stop sanitizing config options

The point of having a whitelist of command-line config
options to pass to submodules was two-fold:

  1. It prevented obvious nonsense like using core.worktree
     for multiple repos.

  2. It could prevent surprise when the user did not mean
     for the options to leak to the submodules (e.g.,
     http.sslverify=false).

For case 1, the answer is mostly "if it hurts, don't do
that". For case 2, we can note that any such example has a
matching inverted surprise (e.g., a user who meant
http.sslverify=true to apply everywhere, but it didn't).

So this whitelist is probably not giving us any benefit, and
is already creating a hassle as people propose things to put
on it. Let's just drop it entirely.

Note that we still need to keep a special code path for
"prepare the submodule environment", because we still have
to take care to pass through $GIT_CONFIG_PARAMETERS (and
block the rest of the repo-specific environment variables).

We can do this easily from within the submodule shell
script, which lets us drop the submodule--helper option
entirely (and it's OK to do so because as a "--" program, it
is entirely a private implementation detail).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>