completion: keep other config var completion in camelCase

completion: keep other config var completion in camelCase

The last patch makes "git config <tab>" shows camelCase names because
that's what's in the source: config.txt. There are still a couple
manual var completion in this code. Let's make them follow the naming
convention as well.

In theory we could automate this part too because we have the
information. But let's stick to one step at a time and leave this for
later.

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