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.