fetch: respect --server-option when fetching multiple remotes

Fix an issue where server options specified via the command line
(`--server-option` or `-o`) were not sent when fetching from multiple
remotes using Git protocol v2.

To reproduce the issue with a repository containing multiple remotes:

  GIT_TRACE_PACKET=1 git -c protocol.version=2 fetch --server-option=demo --all

Observe that no server options are sent to any remote.

The root cause was identified in `builtin/fetch.c:fetch_multiple`, which
is invoked when fetching from more than one remote. This function forks
a `git-fetch` subprocess for each remote but did not include the
specified server options in the subprocess arguments.

This commit ensures that command-line specified server options are
properly passed to each subprocess. Relevant tests have been added.

Signed-off-by: Xing Xin <xingxin.xx@bytedance.com>
Reviewed-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: blanet

builtin/fetch.c

@@ -1981,6 +1981,8 @@ static int fetch_multiple(struct string_list *list, int max_children,
 	strvec_pushl(&argv, "-c", "fetch.bundleURI=",
 		     "fetch", "--append", "--no-auto-gc",
 		     "--no-write-commit-graph", NULL);
+	for (i = 0; i < server_options.nr; i++)
+		strvec_pushf(&argv, "--server-option=%s", server_options.items[i].string);
 	add_options_to_argv(&argv, config);
 
 	if (max_children != 1 && list->nr != 1) {

t/t5702-protocol-v2.sh

@@ -418,6 +418,16 @@ test_expect_success 'server-options are sent when fetching' '
 	grep "server-option=world" log
 '
 
+test_expect_success 'server-options are sent when fetch multiple remotes' '
+	test_when_finished "rm -f log server_options_sent" &&
+	git clone "file://$(pwd)/file_parent" child_multi_remotes &&
+	git -C child_multi_remotes remote add another "file://$(pwd)/file_parent" &&
+	GIT_TRACE_PACKET="$(pwd)/log" git -C child_multi_remotes -c protocol.version=2 \
+		fetch -o hello --all &&
+	grep "fetch> server-option=hello" log >server_options_sent &&
+	test_line_count = 2 server_options_sent
+'
+
 test_expect_success 'server-options from configuration are used by git-fetch' '
 	test_when_finished "rm -rf log myclone" &&
 	git clone "file://$(pwd)/file_parent" myclone &&