cmdutil: add a "changeset.obsolete" label in changeset_printer

Until now there were no label to highlight obsolete changesets in log output,
only evolution troubles (unstable, bumped, divergent) are supported. We add a
"changeset.obsolete" label on changeset entries produced by changeset_printer
so that obsolete changesets can be highlighted in log output. This is useful
because, unless using a graph log where obsolete changesets have a 'x' marker,
there's no way to identify obsolete changesets. And even in graph mode, when
working directory's parent is obsolete, we get a '@' marker and we do not see
it as obsolete.

mercurial/cmdutil.py

@@ -1155,6 +1155,8 b' def diffordiffstat(ui, repo, diffopts, n'
 
 def _changesetlabels(ctx):
     labels = ['log.changeset', 'changeset.%s' % ctx.phasestr()]
+    if ctx.obsolete():
+        labels.append('changeset.obsolete')
     if ctx.troubled():
         labels.append('changeset.troubled')
         for trouble in ctx.troubles():