maintenance: use XDG config if it exists

maintenance: use XDG config if it exists

`git maintenance register` registers the repository in the user's global
config. `$XDG_CONFIG_HOME/git/config` is supposed to be used if
`~/.gitconfig` does not exist. However, this command creates a
`~/.gitconfig` file and writes to that one even though the XDG variant
exists.

This used to work correctly until 50a044f1e4 (gc: replace config
subprocesses with API calls, 2022-09-27), when the command started calling
the config API instead of git-config(1).

Also change `unregister` accordingly.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>