fileset: add missing() predicate (issue4925) Help of status cmd defines status file of 'missing', what is called in fileset 'deleted'. To stay consistent this patch introduces missing() predicate which in fact is alias to 'deleted'.

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.