tests: update more of test-bdiff.py to use unittest (part 3 of 4)

tests: update more of test-bdiff.py to use unittest (part 2 of 4)

tests: migrate test-bdiff.py to use unittest (part 1 of 4) This moves all the test() calls, which were easy and mechanical.

import-checker: do not enforce lexical sort accross stdlib/local boundary Before this change, you could get in a start where the checker would either complain about importing local module before stdlib one or complain about the local one being wrongly lexically sorted with the stdlib one. We detect the boundary and avoid complaining about lexical sort across it.

cg1packer: fix `compressed` method `cg1packer.compressed()` returns True even if `self._type` is 'UN'. This patch fixes it.

perf: add historical support of ui.load() ui.load() has been available since d83ca854 and at the time of writing isn't available on stable branch breaking benchmarking newer stable revisions. Add historical portability policy note on contrib/benchmarks

chg: ignore HG_* in confighash The environment variables `HG_*` are usually used by hooks. Unlike `HGPLAIN` etc, they do not actually affect hg's behavior. So do not include them in confighash. This would avoid spawning an unbound number of chg server processes if commit hook calls hg frequently.

py3: make keys of keyword arguments strings keys of keyword arguments on Python 3 has to be string. We are dealing with bytes in our codebase so the keys are also bytes. Done that using pycompat.strkwargs(). Also after this patch, `hg version` now runs on Python 3.5. Hurray!

error: make it clear that ProgrammingError is for mercurial developers The word "developer" could refer to users - people using hg are likely to be developers. Add adjectives to make it refer to mercurial developers only.

revlog: merge hash checking subfunctions This patch factors the behavior of both methods into 'checkhash'.