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>

t/t1508-at-combinations.sh

@@ -45,7 +45,7 @@ check "@{u}" upstream-two
 check "@{u}@{1}" upstream-one
 check "@{-1}@{u}" master-two
 check "@{-1}@{u}@{1}" master-one
-fail nonsense "@{u}@{-1}"
+nonsense "@{u}@{-1}"
 nonsense "@{1}@{u}"
 
 test_done