fetch: return when parsing submodule.recurse

When parsing config keys, the normal pattern is to return 0 after
completing the logic for a specific config key, since no other key will
match. One instance, for "submodule.recurse", was missing this case in
builtin/fetch.c.

This is a very minor change, and will have minimal impact to
performance. This particular block was edited recently in 56e8bb4fb4
(fetch: use `fetch_config` to store "fetch.recurseSubmodules" value,
2023-05-17), which led to some hesitation that perhaps this omission was
on purpose.

However, no later cases within git_fetch_config() will match the key if
equal to "submodule.recurse" and neither will any key matches within the
catch-all git_default_config().

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -134,6 +134,7 @@ static int git_fetch_config(const char *k, const char *v,
 		int r = git_config_bool(k, v) ?
 			RECURSE_SUBMODULES_ON : RECURSE_SUBMODULES_OFF;
 		fetch_config->recurse_submodules = r;
+		return 0;
 	}
 
 	if (!strcmp(k, "submodule.fetchjobs")) {