show-ref: add --symbolic-name option

For reftable development, it would be handy to have a tool to provide the direct value of any ref whether it be a symbolic ref or not. Currently there is git-symbolic-ref, which only works for symbolic refs, and git-rev-parse, which will resolve the ref. Let's teach show-ref a --symbolic-name option that will cause git-show-ref(1) to print out the value a symbolic reference points to.

fixes: #218