git-hgext: adjust to the lack of `changelog.heads` method

We don't have a `heads` method returning nodeid, but this is very easy to get
the same result.

This was flagged by pytype.

We can note that the fact this code did not break is probably a good sign that
it is dead code.

However this is a question outside of the scop of this series.

hgext/git/gitlog.py

@@ -324,7 +324,7 b' class changelog(baselog):'
         if common is None:
             common = [sha1nodeconstants.nullid]
         if heads is None:
-            heads = self.heads()
+            heads = [self.node(r) for r in self.headrevs()]
 
         common = [self.rev(n) for n in common]
         heads = [self.rev(n) for n in heads]