builtin-config: add --path option doing ~ and ~user expansion.

395de250 (Expand ~ and ~user in core.excludesfile, commit.template)
introduced a C function git_config_pathname, doing ~/ and ~user/
expansion. This patch makes the feature available to scripts with 'git
config --get --path'.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>