completion: Replace config --list with --get-regexp
completion: Replace config --list with --get-regexp

James Bardin noted that the completion spewed warnings when no git config
file is present.  This is likely a bug to be fixed in git config, but it's
also good to simplify the completion code by using the --get-regexp option
as Jeff King pointed out.

Signed-off-by: Todd Zullinger <ef20ee0e31e0ea2e6153df3c6a7c131757c7c986@pobox.com>
Trivially-acked-by: Shawn O. Pearce <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git