config: always parse GIT_CONFIG_PARAMETERS during git_config

config: always parse GIT_CONFIG_PARAMETERS during git_config

Previously we parsed GIT_CONFIG_PARAMETERS lazily into a
linked list, and then checked that list during future
invocations of git_config. However, that ignores the fact
that the environment variable could change during our run
(e.g., because we parse more "-c" as part of an alias).

Instead, let's just re-parse the environment variable each
time. It's generally not very big, and it's no more work
than parsing the config files, anyway.

As a bonus, we can ditch all of the linked list storage code
entirely, making the code much simpler.

The test unfortunately still does not pass because of an
unrelated bug in handle_options.

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