bash completion: Hide more plumbing commands

bash completion: Hide more plumbing commands

git <tab><tab> still shows way too many commands, some of them
are clearly plumbing. This patch hides the plumbing commands
liberally (that is, in special cases, users still might want to
call one of the hidden commands, a *normal* workflow should never
involve these, though - and if it does, we have a UI problem anyway).

Signed-off-by: Petr Baudis <pasky@suse.cz>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>