builtin/gc: inline config values specific to the "files" backend

builtin/gc: inline config values specific to the "files" backend

The `struct gc_config` contains a set of values that we read via the Git
repository's configuration. Several of those values that are consumed by
the object database optimization logic are inherently specific to the
"files" config.

In a later commit we'll make the logic to optimize object databases
pluggable. So by carrying these "files"-backend specific values in the
generic config struct means that other backends would have to worry
about these values, too. This feels somewhat dirty, as implementation-
specific details should live with the backends themselves.

Inline these values directly at the call sites that need them instead.

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