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obsutil: maintain a homogenous list when computing successors

Authored by mharbison72.

Description

obsutil: maintain a homogenous list when computing successors

In practice, this wasn't a problem because of an internal check for an empty
list, but it avoids the following pytype error:

File "/mnt/c/Users/Matt/hg/mercurial/obsutil.py", line 823, in successorsandmarkers: No attribute 'markers' on List[nothing] [attribute-error]
  In Union[Any, List[nothing], _succs]

Differential Revision: https://phab.mercurial-scm.org/D10213