tests/test-dirstate-backup.t

transaction: make undoname a private attribute

This attribute tracks the file pattern to use for undo files.
It is an implementation detail of the current transaction semantics
and doesn't need to be part of the future transaction interface. So
mark it as private.

Differential Revision: https://phab.mercurial-scm.org/D4623

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