Revert "completion: fix shell expansion of items"

Revert "completion: fix shell expansion of items"

This reverts commit 25ae7cfd19c8f21721363c64163cd5d9d1135b20.

That patch does fix expansion of weird variables in some
simple tests, but it also seems to break other things, like
expansion of refs by "git checkout".

While we're sorting out the correct solution, we are much
better with the original bug (people with metacharacters in
their completions occasionally see an error message) than
the current bug (ref completion does not work at all).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>