bash-completion: Add comments to remind about required arguments

bash-completion: Add comments to remind about required arguments

Add a few simple comments above commands that take arguments. These
comments are meant to remind developers of potential problems that
can occur when the script is sourced on systems with "set -u." Any
function which requires arguments really ought to be called with
explicit arguments given.

Also adds a #!bash to the top of bash completions so that editing
software can always identify that the file is of sh type.

Signed-off-by: Ted Pavlic <ted@tedpavlic.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>