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 [off-list ref]
Signed-off-by: Patrick Steinhardt <redacted>
---
 Documentation/git-refs.adoc |   6 ++
 builtin/refs.c              |  49 +++++++++++++++++
 t/meson.build               |   1 +
 t/t1467-refs-rename.sh      | 131 ++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 187 insertions(+)

diff --git a/Documentation/git-refs.adoc b/Documentation/git-refs.adoc
index e6a3528349..ce278c59bf 100644
--- a/Documentation/git-refs.adoc
+++ b/Documentation/git-refs.adoc

@@ -23,6 +23,7 @@ git refs optimize [--all] [--no-prune] [--auto] [--include <pattern>] [--exclude
 git refs create [--message=<reason>] [--no-deref] [--create-reflog] <ref> <new-value>
 git refs delete [--message=<reason>] [--no-deref] <ref> [<old-value>]
 git refs update [--message=<reason>] [--no-deref] [--create-reflog] <ref> <new-value> [<old-value>]
+git refs rename [--message=<reason>] <old-ref> <new-ref>