revlog: drop silly `raw` parameter to `rawdata` function This is a leftover from `revision` and does not make sense for `rawdata`

perf: don't depend on pycompat for older Mercurial versions We already define local _sysstr() and _xrange() for compatibility, so we should use those. Also create a similar local _bytestr() corresponding to pycompat.bytestr(). Differential Revision: https://phab.mercurial-scm.org/D6745