dirstate-v2: Support appending to the same data file
For now we’re still writing the entire data every time, so appending is not
useful yet. Later we’ll have new nodes pointing to some existing data for
nodes and paths that haven’t changed.
The decision whether to append is pseudo-random in order to make tests exercise
both code paths. This will be replaced by a heuristic based on the amount
of unused existing data.
Differential Revision: https://phab.mercurial-scm.org/D11094
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