rev-parse --symbolic-full-name: don't print '^' if SHA1 is not a ref

rev-parse --symbolic-full-name: don't print '^' if SHA1 is not a ref

The intention of --symbolic-full-name is to not print anything if a
revision is not an exact ref. But this command:

    $ git-rev-parse --symbolic-full-name --not master~1

still emitted a sole '^' to stdout (provided that there's no other ref at
master~1). This fixes it.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>