submodule update: run at most one fetch job unless otherwise set

In a028a1930c (fetching submodules: respect `submodule.fetchJobs`
config option, 2016-02-29), we made sure to keep the default
behavior of fetching at most one submodule at once when not setting
the newly introduced `submodule.fetchJobs` config.

This regressed in 90efe595c5 (builtin/submodule--helper: factor
out submodule updating, 2018-08-03). Fix it.

Reported-by: Sjon Hortensius <sjon@parse.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -1551,7 +1551,7 @@ struct submodule_update_clone {
 #define SUBMODULE_UPDATE_CLONE_INIT {0, MODULE_LIST_INIT, 0, \
 	SUBMODULE_UPDATE_STRATEGY_INIT, 0, 0, -1, STRING_LIST_INIT_DUP, 0, \
 	NULL, NULL, NULL, \
-	NULL, 0, 0, 0, NULL, 0, 0, 0}
+	NULL, 0, 0, 0, NULL, 0, 0, 1}
 
 
 static void next_submodule_warn_missing(struct submodule_update_clone *suc,

t/t5526-fetch-submodules.sh

@@ -524,6 +524,8 @@ test_expect_success 'fetching submodules respects parallel settings' '
 	git config fetch.recurseSubmodules true &&
 	(
 		cd downstream &&
+		GIT_TRACE=$(pwd)/trace.out git fetch &&
+		grep "1 tasks" trace.out &&
 		GIT_TRACE=$(pwd)/trace.out git fetch --jobs 7 &&
 		grep "7 tasks" trace.out &&
 		git config submodule.fetchJobs 8 &&