Speed up bash completion loading

Speed up bash completion loading

Since git is not used in each and every interactive xterm, it
seems best to load completion support with cold caches and then
load each needed thing lazily.  This has most of the speed
advantage of pre-generating everything at build time, without the
complication of figuring out at build time what commands will be
available at run time.

On this slow laptop, this decreases the time to load
git-completion.bash from about 500 ms to about 175 ms.

Suggested-by: Kirill Smelkov <kirr@mns.spb.ru>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Cc: Stephen Boyd <bebarino@gmail.com>
Cc: SZEDER Gábor <szeder@ira.uka.de>
Cc: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>