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"
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wrap_comments = true
error_on_line_overflow = true