revlog: code for `revlogv0` in its own module
This code is mostly unused compatiblity code. Yet it take a prohiminent place in
the `revlog.py` module. That module is already quite big, so we move all that
code in a dedicated module.
Differential Revision: https://phab.mercurial-scm.org/D10511
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true