Stop depending on `the_repository` in object-related subsystems

Hi,

this patch series is another step to remove our dependency on the global the_repository variable. The series focusses on subsystems related to objects.

The intent here is to work towards libification of the whole subsystem so that we can start splitting out something like an object "backend". It is thus part of a set of refactorings aimed at allowing pluggable object databases eventually. I'm not discussing that bigger effort yet, mostly because it's still taking shape. So these patch series are things that make sense standalone, even if pluggable ODBs never get to be a thing.

Note that this patch series stop short of dropping the_repository in "object-file.c". This is a bigger undertaking, so I'm pushing that into the next patch series.

Thanks!

Patrick

Closes Refactor `pack-*.c` to not depend on global state (#376 - closed), Refactor `csum-file.c` to not depend on global ... (#507 - closed).