git: make `dirstate.parents()` return a list like the core class

The core class returned a list, so that's how I type annotated it, and this got
flagged.  I suppose we could annotate it as a `Sequence[bytes]`, but it's a
trivial difference.

hgext/git/dirstate.py

@@ -125,7 +125,7 class gitdirstate(intdirstate.idirstate)
 
     def parents(self):
         # TODO how on earth do we find p2 if a merge is in flight?
-        return self.p1(), sha1nodeconstants.nullid
+        return [self.p1(), sha1nodeconstants.nullid]
 
     def __iter__(self):
         return (pycompat.fsencode(f.path) for f in self.git.index)