repo-settings: fix checking for fetch.negotiationAlgorithm=default

repo-settings: fix checking for fetch.negotiationAlgorithm=default

In commit 3050b6dfc75d (repo-settings.c: simplify the setup,
2021-09-21), the branch for handling fetch.negotiationAlgorithm=default
was deleted.  Since this value is documented in
Documentation/config/fetch.txt, restore the check for this value.

Note that this change caused an observable bug: if someone sets
feature.experimental=true in config, and then passes "-c
fetch.negotiationAlgorithm=default" on the command line in an attempt to
override the config, then the override is ignored.  Fix the bug by not
ignoring the value of "default".

Technically, before commit 3050b6dfc75d, repo-settings would treat any
fetch.negotiationAlgorithm value other than "skipping" or "noop" as a
request for "default", but I think it probably makes more sense to
ignore such broken requests and leave fetch.negotiationAlgorithm with
the default value rather than the value of "default".  (If that sounds
confusing, note that "default" is usually the default value, but when
feature.experimental=true, "skipping" is the default value.)

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>