stream-clone: introduce the notion of an experimental "v3" version We introduce a new experimental "v3" stream protocol, disabled by default. In practice the "v3-exp" protocol introduced in this changeset is identical to v2, but this changeset, lay the groundwork for having a new protocol: configuration, capability exchange, test coverage, etc. The actual protocol work will starts in the coming changesets.

stream-clone: check is a compatible protocol can be found The previous code was explicitly checking if "v2" is listed in the "stream" bundle2 capability. The new code is checking is there is anything common between the versions supported client side and server side overlaps. This prepare the introduction of more stream version than "v2".

stream-clone: bail-out earlier if stream clone is not requested The `canperformstreamclone` function is bit messy. However it seems clearer to me to check if a stream-clone have been requested by the client or the server at all, before checking if a compatible protocol can be negotiated with the server. So I am doing some gratuitous movement so reorder conditional.

stream-clone: bail-out earlier if pull is partial The `canperformstreamclone` function is bit messy. However it seems clearer to me to process the very generic condition about "can we consider a stream-clone at all", before checking if a stream-clone is requested and if a compatible protocol can be negotiated with the server. So I am doing some gratuitous movement so reorder conditional.

stream-clone: bail-out earlier if destination repo is not empty The `canperformstreamclone` function is bit messy. However it seems clearer to me to process the very generic condition about "can we consider a stream-clone at all", before checking if a stream-clone is requested and if a compatible protocol can be negotiated with the server. So I am doing some gratuitous movement so reorder conditional.

stream-clone: check the version of streaming clone supported by the client Make the server refuse to produce streaming clone bundle, if the client doesn't specify the stream=v2 capability. This is in preparation to introduce stream=v3. As far as I can tell, this capability was added at the same time as support for bundle2-based streaming pulls was added, so I don't expect clients to break. (the modern client doesn't break, at any rate)

stream-clone: make sure the `stream` capability is set when bundling This is important to start narrowing protocol option in the next changesets.

stream-clone: upgrade the error message for bad stream request The new version if more compact and more consistent with the general Mercurial usage.

stream-clone: yield cache entry in `_entries_walk` too The new function now cover the same ground as _v2_walk.

stream-clone: introduce a _entries_walk That function insert itself between the store and `_v2_walk`. It only deals with StoreEntry unlike `_v2_walk` that deal with actual file. A share of the `_v2_walk` logic will be moved in this new `_entry_walk` function. Having this function will help us to implement a new "v3" version of the protocol that will be more entry centric.