webutil: make _siblings into an object with __iter__ and __len__ _siblings is a helper that is used for displaying changeset parents and children in hgweb. Before, when it was a simple generator, it couldn't tell its length without being consumed, and that required a special case when preparing data for changeset template (see 9e1f4c65f5f5). Let's make it into a class (similar to templatekw._hybrid) that allows len(...) without side-effects.