obsolete: add a function to compute "exclusive-markers" for a set of nodes
This set will be used to select the obsmarkers to be stripped alongside the
stripped changesets. See the function docstring for details.
More advanced testing is introduced in the next changesets to keep this one
simpler. That extra testing provides more example.
#require docutils gettext
$ $TESTDIR/check-gendoc zh_CN
checking for parse errors