Introduce a new command to work with refs

Paraphrasing @pks-gitlab from the mailing list: https://lore.kernel.org/git/ZkNJaaKTTKbns8wo@tanuki/#t

> * for-each-ref, show-ref
> These two commands do very similar things, i.e. list references. Both
> diverge in the features they provided, but its not clear why we have the
> two.

True, they have clear overlap and both are part of plumbing. I never
quite know which one to pick. But git-show-ref(1) handles things that
git-for-each-ref(1) doesn't ("--exists", "--verify") and the other way
round ("--stdin", "--sort=", many more).

Honestly, I think that both of these are not ideal. I think we should
think bigger in this context and introduce a new command with proper
subcommands to make the whole story around refs more coherent and
discoverable:

  # Replaces git-show-ref(1) and git-for-each-ref(1).
  $ git refs list

  # Replaces `git show-ref --exists`.
  $ git refs exist

  # Replaces `git show-ref --verify <ref>`.
  $ git refs show

  # Replaces git-symbolic-ref(1) to show a ref.
  $ git refs resolve

  # Replaces git-pack-ref(1).
  $ git refs pack

  # Replaces git-update-ref(1).
  $ git refs write

  # Replaces git-check-ref-format(1).
  $ git refs check-format

This is of course a much larger topic and something that is very much up
for discussion. But in any case, it indicates that a deprecation would
be premature at this point in time.