you-still-use-that??: help deprecating commands for removal

you-still-use-that??: help deprecating commands for removal

Commands slated for removal like "git pack-redundant" now require
an explicit "--i-still-use-this" option to run.  This is to
discourage casual use and surface their pending deprecation to
users.

The warning message is long, so factor it into a helper function
you_still_use_that() to simplify reuse by other commands.

Also add a missing test to ensure this enforcement works for
"pack-redundant".

Helped-by: Elijah Newren <newren@gmail.com>
[en: log message]
Signed-off-by: Junio C Hamano <gitster@pobox.com>