convert: remove unused dictionaries self.parent, self.lastbranch and self.tags have never been used.

convert: self.heads is a list self.heads is used as a list throughout convert and never a dictionary. Initialize it correctly to a list.

convert: don't use long list comprehensions We are iterating over p4changes. Make the continue condition more clear and easier to add new conditions in future patches, by removing the list comprehension and move the condition into the existing for-loop.