builtin/gc: refactor to read config into structure

The git-gc(1) command knows to read a bunch of config keys to tweak its
own behaviour. The values are parsed into global variables, which makes
it hard to correctly manage the lifecycle of values that may require a
memory allocation.

Refactor the code to use a `struct gc_config` that gets populated and
passed around. For one, this makes previously-implicit dependencies on
these config values clear. Second, it will allow us to properly manage
the lifecycle in the next commit.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t