builtin/fetch: parse recurse-submodules-default at default options parsing

builtin/fetch: parse recurse-submodules-default at default options parsing

Instead of just storing the string and then later calling our own
parsing function 'parse_fetch_recurse_submodules_arg', make use of the
function callback 'option_fetch_parse_recurse_submodules' that was
introduced in the last patch. Also move all submodule recursing variables
in one spot at the top of the file.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>