completion: Use consistent if [...] convention, not "test"

The local coding convention in bash completion is to use [...] rather
than test. Additionally,

if [...]; then

is preferred over

if [...]
then

and so matching "if [...]\nthen" were changed accordingly.

Signed-off-by: Ted Pavlic <ted@tedpavlic.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>