merge: use None as filename for base in 'both created' conflicts

Instead of using a file that we know is not in the common ancestor's
maniffest, let's use None. This is safe as the only place that cares
about the value (applyupdates) already checks if the item exists in
the ancestor.

mercurial/merge.py

@@ -421,9 +421,7 b' def manifestmerge(repo, wctx, p2, pa, br'
                     actions['m'].append((f, (f, f, fa, False, pa.node()),
                                    "both renamed from " + fa))
                 else:
-                    # Note: f as ancestor is wrong - we can't really make a
-                    # 3-way merge without an ancestor file.
-                    actions['m'].append((f, (f, f, f, False, pa.node()),
+                    actions['m'].append((f, (f, f, None, False, pa.node()),
                                    "both created"))
             else:
                 a = ma[f]
@@ -493,8 +491,7 b' def manifestmerge(repo, wctx, p2, pa, br'
             else:
                 different = _checkunknownfile(repo, wctx, p2, f)
                 if force and branchmerge and different:
-                    # FIXME: This is wrong - f is not in ma ...
-                    actions['m'].append((f, (f, f, f, False, pa.node()),
+                    actions['m'].append((f, (f, f, None, False, pa.node()),
                                     "remote differs from untracked local"))
                 elif not force and different:
                     aborts.append((f, 'ud'))