builtin/gc: make repack arguments self-contained

builtin/gc: make repack arguments self-contained

When optimizing the object database most of the heavy-lifting is done by
git-repack(1). The arguments we pass to this function are assembled in
global scope, which is hard to follow.

Refactor the logic by moving the vector into `maintenance_task_odb()`.
While that means we have to pass more arguments to this function, it has
the upside that the logic becomes self-contained without any kind of
global interdependencies.

This is a pure refactoring with no intended functional change.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>