fsck: initialize fsck options via a function

fsck: initialize fsck options via a function

We initialize the `struct fsck_options` via a set of macros, often in
global scope. In the next commit though we're about to introduce a new
repository field to the options that must be initialized, and naturally
we don't have a repo other than `the_repository` available in this
scope.

Refactor the code to instead intrdouce a new `fsck_options_init()`
function that initializes the options for us and move initialization
into function scope.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>