config: factor out config file stack management

config: factor out config file stack management

Because a config callback may start parsing a new file, the
global context regarding the current config file is stored
as a stack. Currently we only need to manage that stack from
git_config_from_file. Let's factor it out to allow new
sources of config data.

Signed-off-by: Heiko Voigt <redacted>
Acked-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>