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.

mercurial/cmdutil.py

@@ -1142,6 +1142,10 b' def diffordiffstat(ui, repo, diffopts, n'
             sub.diff(ui, diffopts, tempnode2, submatch, changes=changes,
                      stat=stat, fp=fp, prefix=prefix)
 
+def _changesetlabels(ctx):
+    labels = ['log.changeset', 'changeset.%s' % ctx.phasestr()]
+    return ' '.join(labels)
+
 class changeset_printer(object):
     '''show changeset information when templating not requested.'''
 
@@ -1202,7 +1206,7 b' class changeset_printer(object):'
 
         # i18n: column positioning for "hg log"
         self.ui.write(_("changeset:   %d:%s\n") % revnode,
-                      label='log.changeset changeset.%s' % ctx.phasestr())
+                      label=_changesetlabels(ctx))
 
         # branches are shown first before any other names due to backwards
         # compatibility