fetch-pack: move fsck options into function scope

fetch-pack: move fsck options into function scope

When fetching a packfile, we optionally verify received objects via the
fsck subsystem. The options for those consistency checks are declared in
global scope without a good reason, and they are never cleaned up. So in
case the options are reused, they may accumulate more state over time.

Furthermore, in subsequent changes we'll introduce a repository pointer
into the structure. Obviously though, we don't have a repository
available at static time, except for `the_repository`, which we don't
want to use here.

Refactor the code to move the options into the respective functions and
properly manage their lifecycle.

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