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

rawdata: update callers in shallowbundle We update callers incrementally because this help bisecting failures. This was useful during development, so we expect it might be useful again in the future.

rawdata: update callers in storageutils We update callers incrementally because this help bisecting failures. This was useful during development, so we expect it might be useful again in the future.

rawdata: update callers in delta utils We update callers incrementally because this help bisecting failures. This was useful during development, so we expect it might be useful again in the future.

rawdata: update callers in repository We update callers incrementally because this help bisecting failures. This was useful during development, so we expect it might be useful again in the future.

rawdata: update callers in testing/storage.py We update callers incrementally because this help bisecting failures. This was useful during development, so we expect it might be useful again in the future.

rawdata: update callers in test-revlog-raw We update callers incrementally because this help bisecting failures. This was useful during development, so we expect it might be useful again in the future.

rawdata: update callers in lfs' tests We update callers incrementally because this help bisecting failures. This was useful during development, so we expect it might be useful again in the future.

rawdata: update callers in lfs' wrapper We update callers incrementally because this help bisecting failures. This was useful during development, so we expect it might be useful again in the future.