gitlog: fix embarassing bug that prevented log from showing correct parents

We had the correct parents in the database, but the changelog was
always returning p1 for all parents. Oops.

Differential Revision: https://phab.mercurial-scm.org/D8263

hgext/git/gitlog.py

@@ -274,7 +274,7 b' class changelog(baselog):'
             if len(c.parents) > 2:
                 raise error.Abort(b'TODO octopus merge handling')
             if len(c.parents) == 2:
-                p2 = self.rev(c.parents[0].id.raw)
+                p2 = self.rev(c.parents[1].id.raw)
         return p1, p2
 
     # Private method is used at least by the tags code.