bundlerepo: use supportedincomingversions instead of allsupportedversions Since bundlerepo is really a pull-like operation, this is the correct method to use here.

bundlerepo: introduce method to find file starts and use it This moves us to the modern iter() technique instead of the `while True` pattern since it's easy. Factored out as a function because I'm about to need this in a second place.