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

In commit 3050b6dfc7 (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 3050b6dfc7, 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>

repo-settings.c

@@ -82,6 +82,8 @@ void prepare_repo_settings(struct repository *r)
 			r->settings.fetch_negotiation_algorithm = FETCH_NEGOTIATION_SKIPPING;
 		else if (!strcasecmp(strval, "noop"))
 			r->settings.fetch_negotiation_algorithm = FETCH_NEGOTIATION_NOOP;
+		else if (!strcasecmp(strval, "default"))
+			r->settings.fetch_negotiation_algorithm = FETCH_NEGOTIATION_DEFAULT;
 	}
 
 	/*

t/t5500-fetch-pack.sh

@@ -927,7 +927,8 @@ test_expect_success 'fetching deepen' '
 	)
 '
 
-test_expect_success 'use ref advertisement to prune "have" lines sent' '
+test_negotiation_algorithm_default () {
+	test_when_finished rm -rf clientv0 clientv2 &&
 	rm -rf server client &&
 	git init server &&
 	test_commit -C server both_have_1 &&
@@ -946,7 +947,7 @@ test_expect_success 'use ref advertisement to prune "have" lines sent' '
 	rm -f trace &&
 	cp -r client clientv0 &&
 	GIT_TRACE_PACKET="$(pwd)/trace" git -C clientv0 \
-		fetch origin server_has both_have_2 &&
+		"$@" fetch origin server_has both_have_2 &&
 	grep "have $(git -C client rev-parse client_has)" trace &&
 	grep "have $(git -C client rev-parse both_have_2)" trace &&
 	! grep "have $(git -C client rev-parse both_have_2^)" trace &&
@@ -954,10 +955,20 @@ test_expect_success 'use ref advertisement to prune "have" lines sent' '
 	rm -f trace &&
 	cp -r client clientv2 &&
 	GIT_TRACE_PACKET="$(pwd)/trace" git -C clientv2 -c protocol.version=2 \
-		fetch origin server_has both_have_2 &&
+		"$@" fetch origin server_has both_have_2 &&
 	grep "have $(git -C client rev-parse client_has)" trace &&
 	grep "have $(git -C client rev-parse both_have_2)" trace &&
 	! grep "have $(git -C client rev-parse both_have_2^)" trace
+}
+
+test_expect_success 'use ref advertisement to prune "have" lines sent' '
+	test_negotiation_algorithm_default
+'
+
+test_expect_success 'same as last but with config overrides' '
+	test_negotiation_algorithm_default \
+		-c feature.experimental=true \
+		-c fetch.negotiationAlgorithm=default
 '
 
 test_expect_success 'filtering by size' '