config: expand tildes in include.path variable

config: expand tildes in include.path variable

You can already use relative paths in include.path, which
means that including "foo" from your global "~/.gitconfig"
will look in your home directory. However, you might want to
do something clever like putting "~/.gitconfig-foo" in a
specific repository's config file.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>