builtin/gc: refactor ODB optimizations to operate on "files" source

builtin/gc: refactor ODB optimizations to operate on "files" source

We have a couple of functions that are implementation details of how the
"files" object database source performs optimizations. These functions
often use global state like `the_repository` and implicitly derive the
source they are supposed to optimize.

Refactor these interfaces to accept a "files" source directly. This will
make it easier to move around the whole logic into "odb/source-files.c"
in a subsequent step.

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