revlog: use separate variables to track version flags
It wasn't obvious to me that "versionflags" is used both to
define the default version+features value for new revlogs and
to track the value read from a revlog.
We rename the former use and add explicit assignment of
"versionflags" later to differentiate between the two.
Differential Revision: https://phab.mercurial-scm.org/D5564