crecord: add skipfolded param to previtem

This just simplifies the API a bit so it matches `nextitem` and I
can handle both nextitem and previtem symmetrically.

mercurial/crecord.py

@@ -161,7 +161,7 b' class patchnode:'
             except AttributeError:  # parent and/or grandparent was None
                 return None
 
-    def previtem(self):
+    def previtem(self, skipfolded=None):
         """
         Try to return the previous item closest to this item, regardless of
         item's type (header, hunk, or hunkline).