rename two internal variables for readability
rename two internal variables for readability

* `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`.

kane-c/pure,depressiveRobot/pure-time,yuya373/pure,joshuarubin/dotfiles,sleep/pure,fvargas/pure,enrdstrm/pure,ZefQ/pure,mcallan83/pure-plus,mafredri/pure,anandubajith/pure,joshuarubin/dotfiles,lavelle/pure,jaredru/zsh-pure,joshuarubin/dotfiles,sindresorhus/pure,perks/pure,joshuarubin/dotfiles,alem0lars/pure,forivall/pure,boushley/pure,btaidm/pure,brandon099/pure,DFurnes/purer,jedahan/pure,supercrabtree/pure,kgrz/pure,KarlKode/pure,dmiedema/pure,runningskull/pure,bramkok/purist