fetch: return when parsing submodule.recurse

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>