When the current branch is renamed, the deletion of the old ref is
recorded in HEAD's log with an empty message. Now that delete_ref()
accepts a reflog message, provide a more descriptive message by
passing along the log message that is given to rename_ref().
The next step will be to extend HEAD's log to also include the second
part of the rename, the creation of the new branch.
Helped-by: Jeff King <peff@peff.net> Signed-off-by: Kyle Meyer <kyle@kyleam.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>