upgrade: move most of revlog.clone method into a _clone method...
upgrade: move most of revlog.clone method into a _clone method
The content of the clone method now focus on parameters validation and
processing. The `_clone` method focus on the actual cloning logic.
Splitting the method out save some indentation and clarify each method code
since it a focussed on one goal.
Differential Revision:
https://phab.mercurial-scm.org/D6899
