completion: simplify __gitcomp and __gitcomp_nl implementations
These shell functions are written in an unnecessarily verbose way;
simplify their "conditionally use $<number> after checking $# against
<number>" logic by using shell's built-in conditional substitution
facilities.
Also remove the first of the two assignments to IFS in __gitcomp_nl
that does not have any effect.
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>