bash: Support unique completion when possible.

Because our use of -o nospace prevents bash from adding a trailing space
when a completion is unique and has been fully completed, we need to
perform this addition on our own.  This (large) change converts all
existing uses of compgen to our wrapper __gitcomp which attempts to
handle this by tacking a trailing space onto the end of each offered
option.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>