streamclone: teach canperformstreamclone to be bundle2 aware We add an argument to canperformstreamclone() to return False if a bundle2 stream clone is available. This will enable the legacy stream clone step to no-op when a bundle2 stream clone is supported. The commented code will be made active when bundle2 supports streaming clone. This patch does foreshadow the introduction of the "stream" bundle2 capability and its "v1" sub-capability. The bundle2 capability mirrors the existing "stream" capability and is needed so clients know whether a server explicitly supports streaming clones over bundle2 (servers up to this point support bundle2 without streaming clone support). The sub-capability will denote which data formats and variations are supported. Currently, the value "v1" denotes the existing streaming clone data format, which I intend to reuse inside a bundle2 part. My intent is to eventually introduce alternate data formats that can be produced and consumed more efficiently. Having a sub-capability means we don't need to introduce a new top-level bundle2 capability when new formats are introduced. This doesn't really have any implications beyond making the capabilities namespace more organized.