revlog: reset _nodepos after strip
When using the pure revlog parser, _nodepos is used to keep track of the
position during index scanning in the non-cached cache. If it is out of
bounds, BaseIndexObject._fix_index will assert. Since strip can actually
remove the position scanned last, make sure to reset it. Add an
assertion in the place where the invariance is clearer.
Differential Revision: https://phab.mercurial-scm.org/D3188
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