revlog: split `rawtext` retrieval out of _revisiondata This part is reasonably independent. Having it on its own clarify the code flow and will help code that inherit from revlog to overwrite specific area only.

revlog: avoid caching raw text too early in _revisiondata Without this change, we could cache the rawtext without considering for it validating the cache or not. If the exception raised by the invalid hash were to be caught and the same revision accessed again, the invalid rawtext would be returned.

revlog: add some documentation to `_revisiondata` code

revlog: move `nullid` early return sooner in `_revisiondata` Let us deal with the special case before we start dealing with more generic case.

revlog: stop calling `basetext` `rawtext` in _revisiondata If the cache entry is used as a base test for delta, it is not the rawtext we need. We update the variable name to clarify this.

revlog: assign rawtext earlier in `_revisiondata` Assigning the revision earlier make the code easier to read.

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

perf: don't try to call `util.queue` on Mercurial version before it existed Differential Revision: https://phab.mercurial-scm.org/D6744

perf: handle NameError for `pycompat.foo` when pycompat wasn't imported On old Mercurial versions, we won't have a pycompat variable defined, and then `pycompat.foo` will raise a NameError. Differential Revision: https://phab.mercurial-scm.org/D6743