config: unify code paths to get global config paths

config: unify code paths to get global config paths

There's two callsites which assemble global config paths, once in the
config loading code and once in the git-config(1) builtin. We're about
to implement a way to override global config paths via an environment
variable which would require us to adjust both sites.

Unify both code paths into a single `git_global_config()` function which
returns both paths for `~/.gitconfig` and the XDG config file. This will
make the subsequent patch which introduces the new envvar easier to
implement.

No functional changes are expected from this patch.

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