nodemap: refresh the persistent data on nodemap creation

The logic to read the data and validate the docket are still in python, so we
need to "help" whatever compiled code live in the index to refresh it.

Otherwise clearing the cache could lead to an expensive full recomputation and
disk update even when the persisted data are still valid.

Differential Revision: https://phab.mercurial-scm.org/D8174
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