Re: [PATCH v5 3/3] config: allow overriding of global and system configuration

> In order to have git run in a fully controlled environment without any
> misconfiguration, it may be desirable for users or scripts to override
> global- and system-level configuration files. We already have a way of
> doing this, which is to unset both HOME and XDG_CONFIG_HOME environment
> variables and to set `GIT_CONFIG_NOGLOBAL=true`. This is quite kludgy,
> and unsetting the first two variables likely has an impact on other
> executables spawned by such a script.
> 
> The obvious way to fix this would be to introduce `GIT_CONFIG_NOGLOBAL`
> as an equivalent to `GIT_CONFIG_NOSYSTEM`. But in the past, it has
> turned out that this design is inflexible: we cannot test system-level
> parsing of the git configuration in our test harness because there is no
> way to change its location, so all tests run with `GIT_CONFIG_NOSYSTEM`
> set.
> 
> Instead of doing the same mistake with `GIT_CONFIG_NOGLOBAL`, introduce
> two new variables `GIT_CONFIG_GLOBAL` and `GIT_CONFIG_SYSTEM`:
> 
>     - If unset, git continues to use the usual locations.
> 
>     - If set to a specific path, we skip reading the normal
>       configuration files and instead take the path. By setting the path
>       to `/dev/null`, no configuration will be loaded for the respective
>       level.
> 
> This implements the usecase where we want to execute code in a sanitized
> environment without any potential misconfigurations via `/dev/null`, but
> is more flexible and allows for more usecases than simply adding
> `GIT_CONFIG_NOGLOBAL`.

> +test_expect_success 'write to overridden global and system config' '
> +	cat >expect <<EOF &&
> +[config]
> +	key = value
> +EOF
> +
> +	GIT_CONFIG_GLOBAL=write-to-global git config --global config.key value &&
> +	test_cmp expect write-to-global &&
> +
> +	GIT_CONFIG_SYSTEM=write-to-system git config --system config.key value &&
> +	test_cmp expect write-to-system
> +'