path.c: and an option to call real_path() in expand_user_path()

In the next patch we need the ability to expand '~' to
real_path($HOME). But we can't do that from outside because '~' is part
of a pattern, not a true path. Add an option to expand_user_path() to do
so.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>