completion: use ls -1 instead of rolling a loop to do that ourselves

This simplifies the code a great deal.  In particular, it allows us to
get rid of __git_shopt, which is used only in this fuction to enable
'nullglob' in zsh.

[jn: squashed with a patch that actually gets rid of __git_shopt]

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>