revset: turn spanset into a factory function We rename the `spanset` class to `_spanset`. `spanset` is now a function that builds either a `fullreposet` or a `_spanset` according to the argument passed. At some point, we may force people to explicitly use the `fullreposet` constructor, but the current approach makes it easier to ensure we use the new class whenever possible and focus on the benefits of this class.

revert: add a fullreposet class Every revset evaluation starts from `subset = spanset(repo)` and a lot of revset predicates build a `spansetrepo` for their internal needs. `spanset` is a generic class that can handle any situation. As a result a lot of operation between spanset result in an `orderedlazyset`, a safe object but suboptimal in may situation. So we introduce a `fullreposet` class where some of the operation will be overwritten to produce more interesting results.

obsolete: ensure that `getrevs` always return a set When a set of revisions was empty, we were using an empty tuple. We now return an empty frozenset to ensure the object could be used in an operation that requires a set.

hgweb: fail if an invalid command was supplied in url path (issue4071) Traditionally, the way to specify a command for hgweb was to use url query arguments (e.g. "?cmd=batch"). If the command is unknown to hgweb, it gives an error (e.g. "400 no such method: badcmd"). But there's also another way to specify a command: as a url path fragment (e.g. "/graph"). Before, hgweb was made forgiving (looks like it was made in 44c5157474e7) and user could put any unknown command in the url. If hgweb couldn't understand it, it would just silently fall back to the default command, which depends on the actual style (e.g. for paper it's shortlog, for monoblue it's summary). This was inconsistent and was breaking some tools that rely on http status codes (as noted in the issue4071). So this patch changes that behavior to the more consistent one, i.e. hgweb will now return "400 no such method: badcmd". So if some tool was relying on having an invalid command return http status code 200 and also have some information, then it will stop working. That is, if somebody typed foobar when they really meant shortlog (and the user was lucky enough to choose a style where the default command is shortlog too), that fact will now be revealed. Code-wise, the changed if block is only relevant when there's no "?cmd" query parameter (i.e. only when command is specified as a url path fragment), and looks like the removed else branch was there only for falling back to default command. With that removed, the rest of the code works as expected: it looks at the command, and if it's not known, raises a proper ErrorResponse exception with an appropriate message. Evidently, there were no tests that required the old behavior. But, frankly, I don't know any way to tell if anyone actually exploited such forgiving behavior in some in-house tool.