fix parsing of @{-1}@{u} combination

Previously interpret_branch_name would see @{-1} and stop
parsing, leaving the @{u} as cruft that provoked an error.
Instead, we should recurse if there is more to parse.

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

t/t1508-at-combinations.sh

@@ -43,8 +43,8 @@ check "@{-1}" old-two
 check "@{-1}@{1}" old-one
 check "@{u}" upstream-two
 check "@{u}@{1}" upstream-one
-fail check "@{-1}@{u}" master-two
-fail check "@{-1}@{u}@{1}" master-one
+check "@{-1}@{u}" master-two
+check "@{-1}@{u}@{1}" master-one
 fail nonsense "@{u}@{-1}"
 nonsense "@{1}@{u}"