revlog: use %d to format int instead of %lu (issue6565) The issue says gcc warns that the data types don't match. I couldn't reproduce the warning locally for some reason, but this patch shouldn't break things. Maybe %lu was simply a copy-paste error from 6b1eae313b2f (https://phab.mercurial-scm.org/D10625).

rhg: correctly handle the case where diffs are encoded relative to nullrev returning a valid entry for nullrev fix chain that delta against nullrev.

test-revlog: test a repository that contains a diff against nullrev We are witnessing a crash in the rust code, so we lets make sure this case is tested.

worker: adapt _blockingreader to work around a python3.8.[0-1] bug (issue6444) Python 3.8.0 is the latest I can load on Ubuntu 18.04, and I regularly hit the TypeError because this function is missing. While it can be avoided by disabling worker usage via config option, that's a bit obscure. I'm limiting the function definition to the narrow range of affected pythons because there were other bugs in this area that were worked around, that I don't fully understand. See the bug report for discussions on why the narrow range, and related commits working around other bugs. Differential Revision: https://phab.mercurial-scm.org/D12627