cmdutil: extract a _changesetlabels function out of changeset_printer._show()
There is a common logic in changeset_printer and in the summary command for
labelling a changeset.
This prepares extension of changeset's labels with evolution "troubles"
information that would show up in both log and summary outputs. Ultimately,
both would use this function.
% creating repo
% add and commit
C test.py
% change
M test.py