Pass "git config" flags in the right order

New enough gits accept the --null and --list flags in any order, but
older gits want --null before --list. So do it the way that's fine
with everyone.

I think (but haven't verified) that git started to accept the flags in
any order after commit d64ec16c2af4ddcf3985d11d5dc28a15db181de5 ("git
config: reorganize to use parseopt") by Felipe Contreras on
2009-02-21, first released in git v1.6.3.

Signed-off-by: Karl Hasselström <kha@treskal.com>