revlog: use "entry_point" phrasing for loading the revlog The main entry for the revlog will not necessary be the index, but a small "docket". So we change the variable names and we move the initialisation of the index_file and data_file after that first entry point have been read. Differential Revision: https://phab.mercurial-scm.org/D10601

revlog: directly use self._format_flags when loading index The shorthand variable does not bring much, so we drop it to simplify the code. Differential Revision: https://phab.mercurial-scm.org/D10600

revlog: directly use self._format_version when loading index The shorthand variable does not bring much, so we drop it to simplify the code. Differential Revision: https://phab.mercurial-scm.org/D10599

revlog: use `_format_flags` to access flags instead of `header` It seems better to reuse the variable we carefully extracted This also open the way to more flexible way to retrieve these flags. Differential Revision: https://phab.mercurial-scm.org/D10598

revlog: rename `newversionflags` to `new_header` This make it consistent with the previous changeset. Differential Revision: https://phab.mercurial-scm.org/D10597

revlog: rename `versionflags` to header The variable is named as such because it contains "version" information and "flags" information. However you mostly needs to know the code to understand the name which is not great. The fact that this is the very first four bytes in all revlog seems more relevant, so we rename the variable "header". Differential Revision: https://phab.mercurial-scm.org/D10596

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