commands: support hg help scripting.HGPLAIN
keywords are lowercased within help/minirst...
run-tests: use $HGTEST_RUN_TESTS_PURE
test-run-tests and test-hghave call run-tests;
if you don't have a working build environment, and you are trying
to use --pure, then if they don't use --pure or --with-hg,
they'll break.
minirst: don't treat top level item as children of last item (
issue4803)
test for "hg help -c config" in help config