archive: complain about path specs that don't match anything

archive: complain about path specs that don't match anything

Verify that all path specs match at least one path in the specified
tree and reject those that don't.

This would have made the bug fixed by 782a0005 easier to find.

This implementation is simple to the point of being stupid.  It walks
the full tree for each path spec until it matches something.  It's short
and seems to be fast enough, though.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>