If history file didn't exist when irb was started, @loaded_history_mtime
would be nil. However, if the history file didn't exist before, but it
exists when saving history, that means the history file was modified,
and we should handle it the same way as we handle the other case where
the history file was modified.
Fixes#388https://github.com/ruby/irb/commit/8d277aafcb