config: use '_config' within 'configbool' This will prevent bugs from using None as the sentinel value (eg: 'ui.interactive')

config: extract the core config logic into a private method This will make it easier for the other 'configxxx' function to detect unset value.

debugrevlog: also display the largest delta chain span Mercurial read all data between the base of the chain and the last delta when restoring content (including unrelated delta). To monitor this, we add data about the size of the "delta chain span" to debugrevlog.