nodemap: add a test about racy commit during stream clone
That test show that the resulting client nodemap is different from the server
one. This happens because the server one transferred a corrupted node map. The
data file match the pre-commit content while the docket has post commit content.
As the result the nodemap was detected invalid and recomputed.
When running without the rust implementation, the code is also generating a new
datafile unconditionally, This mean the older file is no longer there are
transfer time, resulting in a crash.
We will fix this issue later, but we start with writing tests highlighting the
issue.
Differential Revision: https://phab.mercurial-scm.org/D10479
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