revert: no backup for `dsadded` set

There is only one case where a backup is required in the `dsadded` set, and the
current backup mechanism fails to handle it. So we stop trying to do backups at
all for now. This will help us to simplify the backup code and finally fix
this backup issue.

mercurial/cmdutil.py

@@ -2525,7 +2525,7 b' def revert(ui, repo, ctx, parents, *pats'
             # Modified compared to target, local change
             (dsmodified,    actions['revert'],   backup),
             # Added since target
-            (dsadded,       actions['remove'],   backup),
+            (dsadded,       actions['remove'],   discard),
             # Removed since  target, before working copy parent
             (removed,       actions['add'],      backup),
             # Removed since targe, marked as such in working copy parent
@@ -2537,7 +2537,7 b' def revert(ui, repo, ctx, parents, *pats'
             (unknown,       actions['unknown'],  discard),
             )
 
-        needdata = ('revert', 'add', 'remove', 'undelete')
+        needdata = ('revert', 'add', 'undelete')
         _revertprefetch(repo, ctx, *[actions[name][0] for name in needdata])
 
         for abs, (rel, exact) in sorted(names.items()):