builtin/config: move "fixed-value" option to correct group

builtin/config: move "fixed-value" option to correct group

The `--fixed-value` option can be used to alter how the value-pattern
parameter is interpreted for the various actions of git-config(1). But
while it is an option, it is currently listed as part of the actions
group, which is wrong.

Move the option to the "Other" group, which hosts the various options
known to git-config(1).

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>