context: add a changectx.diff() convenience function

With this function, extracting diffs becomes trivial:

repo = hg.repository(ui.ui(), path=root)
ctx = repo['tip']
for out in ctx.diff():  print out

mercurial/context.py

@@ -7,7 +7,7 b''
 
 from node import nullid, nullrev, short, hex
 from i18n import _
-import ancestor, bdiff, error, util, subrepo
+import ancestor, bdiff, error, util, subrepo, patch
 import os, errno
 
 propertycache = util.propertycache
@@ -204,6 +204,14 b' class changectx(object):'
     def sub(self, path):
         return subrepo.subrepo(self, path)
 
+    def diff(self, ctx2=None, match=None):
+        """Returns a diff generator for the given contexts and matcher"""
+        if ctx2 is None:
+            ctx2 = self.p1()
+        if ctx2 is not None and not isinstance(ctx2, changectx):
+            ctx2 = self._repo[ctx2]
+        return patch.diff(self._repo, ctx2.node(), self.node(), match=match)
+
 class filectx(object):
     """A filecontext object makes access to data related to a particular
        filerevision convenient."""