bash_completion: Always enable extglob

In older versions of GNU Bash extended patterns, such as “@(…)” are only
available with the “extglob” shell option. This patch adds a wrapper
function so that extglob is always enabled while doing any completion.
Due to early returns inside the main completion function this seemed
like the best option.

Reported by Sascha Lucas.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>