tests: add tests and document expectations from visit_children_set in rust The tests this patch are adding have the form of formal spec in invariants::visit_children_set::holds, and then a series of checks that all examples must satisfy this formal spec. I tried to make the spec consistent with how this function is used and how it was originally conceived. This is in conflict with how it's documented in Rust. Some of the implementations also fail to implement this spec, which leads to bugs, in particular when complicated patterns are used with `hg status`.

tests: add a test that demonstrates a bug in rhg status pattern handling The bug is in [visit_children_set], will be elaborated on in follow-up changes.

bundle-spec: properly parse boolean configuration as boolean Before this changesets "v2;revbranchcache=no" would actually request the addition for a revbranchcache part as the non-empty string `"0"` is `True`

bundle-spec: properly identify changegroup-less bundle It is possible to produce a bundle without changegroup. For example if we want to only send phases or obsolescence information. However that lead to crash for command that identifies bundle content. So we fix that. The test will come in the next changesets, when we fix another bug preventing to generate such bundle by hand.