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.

hgext/convert/p4.py

@@ -56,7 +56,7 b' class p4_source(common.converter_source)'
         common.checktool('p4', abort=False)
 
         self.p4changes = {}
-        self.heads = {}
+        self.heads = []
         self.changeset = {}
         self.files = {}
         self.copies = {}