revlog: move index reading logic in a dedicated method They are multiple motivation to do it: * The logic is complicated enough to deserver its own method. * We will need to reuse this once we put a docket in use. * This split the actual reading from the processing of the read data better. Differential Revision: https://phab.mercurial-scm.org/D10595

revlog: define the actual index and datafile at loading time This is just code movement, to make the code closer to where we actually use it and where it will be defined in the future. Differential Revision: https://phab.mercurial-scm.org/D10594

revlog: simplify a conditionnal in _enforceinlinesize This is a gratuitous change to make things a bit easier to read. Differential Revision: https://phab.mercurial-scm.org/D10593