Support config include using libgit

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Add routines to get config filenames in UTF-8 and forward slashes

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Use the routines to get config filenames in UTF-8 and forward slashes to libgit2

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Pass system config filenames in forward slashes to libgit2

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>