completion: normalize increment/decrement style

completion: normalize increment/decrement style

The style used for incrementing and decrementing variables was fairly
inconsistenty and was normalized to use x++, or ((x++)) in contexts
where the former would otherwise be interpreted as a command. This is a
bash-ism, but for obvious reasons this script is already bash-specific.

Signed-off-by: Philip Jägenstedt <philip@foolip.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>