git: add `deprecated` category to --list-cmds

git: add `deprecated` category to --list-cmds

With 145 builtin commands (according to `git --list-cmds=builtins`),
users are probably not keeping on top of which ones (if any) are
deprecated.

Let’s expand the experimental `--list-cmds`[1] to allow users and
programs to query for this information.  We will also use this in an
upcoming commit to implement `is_deprecated_command`.

[1]: Using something which is experimental to query for deprecations is
    perhaps not the most ideal approach, but it is simple to implement
    and better than having to scan the documentation

Acked-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>