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
== 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 ==