obsolete: function and method to access some obsolete data

An `obsolete` boolean property is added to changeset context. Function to get
obsolete marker object from a changeset context are added to the obsolete
module.

mercurial/context.py

@@ -230,6 +230,10 b' class changectx(object):'
         for d in self._repo.changelog.descendants([self._rev]):
             yield changectx(self._repo, d)
 
+    def obsolete(self):
+        """True if the changeset is obsolete"""
+        return self.node() in self._repo.obsstore.obsoleted
+
     def _fileinfo(self, path):
         if '_manifest' in self.__dict__:
             try:

mercurial/obsolete.py

@@ -267,3 +267,13 b' def allmarkers(repo):'
     for markerdata in repo.obsstore:
         yield marker(repo, markerdata)
 
+def precursormarkers(ctx):
+    """obsolete marker making this changeset obsolete"""
+    for data in ctx._repo.obsstore.precursors.get(ctx.node(), ()):
+        yield marker(ctx._repo, data)
+
+def successormarkers(ctx):
+    """obsolete marker marking this changeset as a successors"""
+    for data in ctx._repo.obsstore.successors.get(ctx.node(), ()):
+        yield marker(ctx._repo, data)
+