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
== New Features ==
* `hg config` now has a `--source` option to show where each
configuration value comes from.
== Default Format Change ==
These changes affects newly created repositories (or new clone) done with
Mercurial 5.8.
== New Experimental Features ==
== Bug Fixes ==
== Backwards Compatibility Changes ==
== Internal API Changes ==