builtin/gc: extract object database optimizations into separate function

builtin/gc: extract object database optimizations into separate function

Extract the object database optimization logic from `cmd_gc()` into a
new `maintenance_task_odb()` helper function. This is a pure refactoring
with no intended functional change.

Note that the message that notifies the user about too many loose
objects is moved into the new function, as well. It is inherently an
implementation detail of how the "files" source works, and as a
consequence we'll move it around in a later commit, as well. This
reordering means that the warning may now be printed at a different
point in time, but it's not expected that this will have any practical
implications.

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