Expand ~ and ~user in core.excludesfile, commit.template

Expand ~ and ~user in core.excludesfile, commit.template

These config variables are parsed to substitute ~ and ~user with getpw
entries.

user_path() refactored into new function expand_user_path(), to allow
dynamically allocating the return buffer.

Original patch by Karl Chen, modified by Matthieu Moy, and further
amended by Junio C Hamano.

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