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>

usage.c

@@ -372,3 +372,15 @@ void bug_fl(const char *file, int line, const char *fmt, ...)
 	trace2_cmd_error_va(fmt, ap);
 	va_end(ap);
 }
+
+NORETURN void you_still_use_that(const char *command_name)
+{
+	fprintf(stderr,
+		_("'%s' is nominated for removal.\n"
+		  "If you still use this command, please add an extra\n"
+		  "option, '--i-still-use-this', on the command line\n"
+		  "and let us know you still use it by sending an e-mail\n"
+		  "to <git@vger.kernel.org>.  Thanks.\n"),
+		command_name);
+	die(_("refusing to run without --i-still-use-this"));
+}