context: enforce memctx.files() unicity

localrepo.commitctx() expects changed files lists to contain unique items and
does not sort itself. Passing duplicates makes them evaluated more than once
and changes the committed changelog id.

mercurial/context.py

@@ -734,7 +734,7 b' class memctx(object):'
         parents = [(p or nullid) for p in parents]
         p1, p2 = parents
         self._parents = [changectx(self._repo, p) for p in (p1, p2)]
-        files = util.sort(list(files))
+        files = util.sort(util.unique(files))
         self._status = [files, [], [], [], []]
         self._filectxfn = filectxfn