convert: do not provide head revisions if we have no changests to import

Don't set a head revision in cases where we have a revmap but no
changesets to import, as convertcmd.convert() treats them as heads of
to-imported revisions.

hgext/convert/p4.py

@@ -228,7 +228,7 b' class p4_source(common.converter_source)'
             self.copies[change] = copies
             lastid = change
 
-        if lastid:
+        if lastid and len(self.changeset) > 0:
             self.heads = [lastid]
 
     def getheads(self):