bash: Support unique completion on git-config.

bash: Support unique completion on git-config.

In many cases we know a completion will be unique, but we've disabled
bash's automatic space addition (-o nospace) so we need to do it
ourselves when necessary.

This change adds additional support for new configuration options
added in 1.5.0, as well as some extended completion support for
the color.* family of options.

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