stream-clone: support streamv3 on the cli [hg bundle] We add support and test for this. The support is still experimental, so the various name and identifier will eventually need to be renamed when stream-v3 gets out of experimental.

stream-clone: add the `-exp` prefix to the bundle part We forget to do so in 58adcabc295f, however this is important to prevent "current" client to send incompatible version to future client.

zstd: hack include order to ensure that our zstd.h is found If the regular Python CFLAGS include directories that already have the zstd headers available, a different and possible incompatible version can be picked up otherwise. Sadly, it seems like Python has no easy way to prefix flags before the rest.

exchange: allow passing no includes/excludes to `pull()` I would expect that `exchange.pull(includepats=[])` results in an empty list of include patterns to be passed to the remote, but it doesn't currently because we check for any truthy value instead of checking specifically for `not None`.

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.