exchange: refactor APIs to obtain bundle data (API) Currently, exchange.getbundle() returns either a cg1unpacker or a util.chunkbuffer (in the case of bundle2). This is kinda OK, as both expose a .read() to consumers. However, localpeer.getbundle() has code inferring what the response type is based on arguments and converts the util.chunkbuffer returned in the bundle2 case to a bundle2.unbundle20 instance. This is a sign that the API for exchange.getbundle() is not ideal because it doesn't consistently return an "unbundler" instance. In addition, unbundlers mask the fact that there is an underlying generator of changegroup data. In both cg1 and bundle2, this generator is being fed into a util.chunkbuffer so it can be re-exposed as a file object. util.chunkbuffer is a nice abstraction. However, it should only be used "at the edges." This is because keeping data as a generator is more efficient than converting it to a chunkbuffer, especially if we convert that chunkbuffer back to a generator (as is the case in some code paths currently). This patch refactors exchange.getbundle() into exchange.getbundlechunks(). The new API returns an iterator of chunks instead of a file-like object. Callers of exchange.getbundle() have been updated to use the new API. There is a minor change of behavior in test-getbundle.t. This is because `hg debuggetbundle` isn't defining bundlecaps. As a result, a cg1 data stream and unpacker is being produced. This is getting fed into a new bundle20 instance via bundle2.writebundle(), which uses a backchannel mechanism between changegroup generation to add the "nbchanges" part parameter. I never liked this backchannel mechanism and I plan to remove it someday. `hg bundle` still produces the "nbchanges" part parameter, so there should be no user-visible change of behavior. I consider this "regression" a bug in `hg debuggetbundle`. And that bug is captured by an existing "TODO" in the code to use bundle2 capabilities.