st: Update to godir.ado (cd to data directory)

["Allan Reese (Cefas)" <allan.reese@cefas.co.uk>]

Thanks to Dan Blanchette, a much improved little program to change
directory to one from which you've just -use-d a dataset.  A suggestion
to StataCorp would be to add a -cd- option to -use-, so -use
"blah/thisdata, cd" would load the data and make blah the current
directory.

program godir
  version 8
  if strpos(reverse("$S_FN"),"/") {
     local lastslash = length("$S_FN")-strpos(reverse("$S_FN"),"/")
   }
   else {
     local lastslash = length("$S_FN")-strpos(reverse("$S_FN"),"\")
   }
  local target = substr("$S_FN", 1,`lastslash')
  if strpos("`target'", ".dta") local target= "`c(pwd)'"
  cd "`target'"
  end
  



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