Avoid Markdown-folding in diff mode

[KSR-Yasuda]

Fix for #427.

[KSR-Yasuda]

Off-topic: some tests do not work correctly in non-English environment?

For example, fold text result test in test/folding-toc.vader,
it expects +-- 28 lines: hello in the folding marker line, though,
Japanese environment shows +-- 28 行: hello, and the test fails.

Tried to fix it with setting $LANG, $LANGUAGE, and $LC_ALL environment variables in test/run-tests.sh,
but it did not have any effect.
I gave up.