builtin/refs: add "rename" subcommand

builtin/refs: add "rename" subcommand

Add a "rename" subcommand to git-refs(1) with the syntax:

  $ git refs rename <oldref> <newref>

It renames <oldref> together with its reflog to <newref>; even when used
on a local branch ref, the current value and the reflog of the ref are
the only things that are renamed. Document it and redirect casual users
to "git branch -m" if that is what they wanted to do.

Co-authored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>