revset: add experimental ancestors/descendants relation subscript The relation name is 'generations' now, which may be changed in future.

revset: add experimental relation and subscript operators The proposed syntax [1] was originally 'set{n rel}', but it seemed slightly confusing if template is involved. On the other hand, we want to keep 'set[n]' for future extension. So this patch introduces 'set#rel[n]' ternary operator. I chose '#' just because it looks like applying an attribute. This also adds stubs for 'set[n]' and 'set#rel' operators since these syntax elements are fundamental for constructing 'set#rel[n]'. [1]: https://www.mercurial-scm.org/wiki/RevsetOperatorPlan#ideas_from_mpm

revset: do not compute weight for integer literal argument In x^n and x~n, n isn't a set expression. There's no need to optimize the right-hand side.

templatekw: export ui.paths as {peerpaths} It's sometimes useful to show hyperlinks in log output. "{get(peerpaths, "default")}/rev/{node}" Since each path may have sub options, "{peerpaths}" is structured as a dict of dicts, but the inner dict is rendered as if it were a string URL. The implementation is ad-hoc, so there are some weird behaviors described in the test. We might need to introduce a proper way of handling a hybrid scalar object. This patch adds _hybrid.__getitem__() so d['path']['url'] works. The keyword is named as "peerpaths" since "paths" seemed too generic in log context.