reject @{-1} not at beginning of object name

reject @{-1} not at beginning of object name

Something like foo@{-1} is nonsensical, as the @{-N} syntax
is reserved for "the Nth last branch", and is not an actual
reflog selector. We should not feed such nonsense to
approxidate at all.

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