code: def findfile(self, old, new):
      
      if exists(old):
        return old
      elif exists(new):
        return new
      else:
        # [w] Google Code generates broken patches with its online editor
        debug("broken patch from Google Code, stripping prefixes..")
        if old.startswith(b'a/') and new.startswith(b'b/'):
          old, new = old[2:], new[2:]
          debug("   %s" % old)
          debug("   %s" % new)
          if exists(old):
            return old
          elif exists(new):
            return new
        return None
docs: return name of file to be patched or None