Remove more sed invocations from within bash completion.

Remove more sed invocations from within bash completion.

This change removes between 1 and 4 sed invocations per completion
entered by the user.  In the case of cat-file the 4 invocations per
completion can take a while on Cygwin; running these replacements
directly within bash saves some time for the end user.

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