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.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL