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
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors