Draft: show-ref: add --show-symrefs 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 add a --show-symrefs option that will print out the value of what a symref points to if it matches the .