webutil: make _siblings into an object with __iter__ and __len__...
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.
