.arcconfig

streamclone: treat volatile file as "fullfile"

The nodemap related file might change (or get deleted) during the stream clone
in a way incompatible with the streaming process.

So we introduce a new flag for this type of file and integrate it with the
existing `revlog_type` field recently added to `store.walk` returns.

We use that new flat to dispatch such file to the existing mechanism for "atomic
replacement" file for the nodemap docket and datafile. This fix the bugs we have
been adding tests for.

Strictly speaking, the nodemap datafile is happened only a could maybe be used
in a slightly more efficient way, however this is good enough for now.

Differential Revision: https://phab.mercurial-scm.org/D10481

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