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.