tests/test-dirstate-backup.t

histedit: conditionalize the imports of 'fcntl' and 'termios'

The recent import of chistedit in c36175456350 made Windows sad.  I'm not sure
if there's other stuff that needs to be done here (e.g. change the default
interface), but this makes the tests run again.

It would have been nicer if the error message indicated these modules were the
problem, but instead it said "*** failed to import extension histedit: No module
named histedit".  I'm not sure if there's anything we can do about that.

Set up

  $ hg init repo
  $ cd repo

Try to import an empty patch

  $ hg import --no-commit - <<EOF
  > EOF
  applying patch from stdin
  abort: stdin: no diffs found
  [255]

No dirstate backups are left behind

  $ ls .hg/dirstate* | sort
  .hg/dirstate