streamclonebundle: make sure we accept new stream clone bundle spec
When asked specifically, the code do a sanity check on the clone bundle to
ensure it's a stream bundle. As we introduced a new stream bundle spec, update
the logic to support it.
With this final changeset, we can now announce safely a stream v2 clone bundle
and old clients would not crash trying to process it.
This changeset address the last comment from Gregory Szorc on the stream v2
bundle series.
Differential Revision: https://phab.mercurial-scm.org/D1957
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