builtin-config: Fix crash when using "-f <relative path>" from non-root dir

builtin-config: Fix crash when using "-f <relative path>" from non-root dir

When your current directory is not at the root of the working tree, and you
use the "-f" option with a relative path, the current code tries to read
from a wrong file, since argv[2] is now beyond the end of the rearranged
argument list.

This patch replaces the incorrect argv[2] with the variable holding the
given config file name.

The bug was introduced by d64ec16 (git config: reorganize to use parseopt).

[jc: added test]

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>