config: move Git config parsing into "environment.c"

config: move Git config parsing into "environment.c"

In "config.c" we host both the business logic to read and write config
files as well as the logic to parse specific Git-related variables. On
the one hand this is mixing concerns, but even more importantly it means
that we cannot easily remove the dependency on `the_repository` in our
config parsing logic.

Move the logic into "environment.c". This file is a grab bag of all
kinds of global state already, so it is quite a good fit. Furthermore,
it also hosts most of the global variables that we're parsing the config
values into, making this an even better fit.

Note that there is one hidden change: in `parse_fsync_components()` we
use an `int` to iterate through `ARRAY_SIZE(fsync_component_names)`. But
as -Wsign-compare warnings are enabled in this file this causes a
compiler warning. The issue is fixed by using a `size_t` instead.

This change allows us to drop the `USE_THE_REPOSITORY_VARIABLE`
declaration.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>