memctx: make `parents()` return list of one element if it’s not a merge

It is part of the contract of `parents()` that non-merges return a list of one
element. `self._parents` is always a list of two elements in `memctx`.

Differential Revision: https://phab.mercurial-scm.org/D8731

mercurial/context.py

@@ -2891,6 +2891,11 b' class memctx(committablectx):'
 
         return scmutil.status(modified, added, removed, [], [], [], [])
 
+    def parents(self):
+        if self._parents[1].node() == nullid:
+            return [self._parents[0]]
+        return self._parents
+
 
 class memfilectx(committablefilectx):
     """memfilectx represents an in-memory file to commit.