stabletailgraph: add test cases challenging the open merge stack This adds three more complex test cases with situations requiring tricky state update in the stack-based iteration (arriving soon).

stabletailgraph: naive version of leap computation This adds a naive reference implementation of the computation of leap and specific leap sets (described in the code documentation). The existing tests are enriched accordingly.

stabletailgraph: extract _parents util func

stabletailgraph: clarify excl part size computation

stabletailgraph: clarify naiveness of current implementation Both the naive and the actual versions of the algorithms are going to co-exist for the tests. This makes is clearer that this one is naive.

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.