.arcconfig

bundle2: don't advertise stream bundle2 capability when feature disabled

The server.uncompressed config option can be used to disable streaming
clones. While the top-level capability to advertise streaming clone
support isn't advertised when this option is set, we were still sending
the bundle2-level capabilities advertising support for stream parts.
It makes sense to not advertise that support when streaming clones
are globally disabled.

If the structure of the new code seems a bit odd, it is because we'll
have to further tweak the behavior in commit(s) ahead.

Differential Revision: https://phab.mercurial-scm.org/D1929

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