* `prompt_pure_git_delay_dirty_check` holds the (finish) timestamp of
the last dirty check (if the dirty check took more than 2 seconds);
renamed to `prompt_pure_git_last_dirty_check_timestamp`.
* Local variable `dirty_check` holds the elapsed time in seconds since
the last dirty check (if the timestamp is set); renamed to
`time_since_last_dirty_check`.
