Org reader: handle minlevel option differently.

[leungbk]

When minlevel exceeds the original minimum level observed in the
file to be included, every heading should be shifted rightward.

Closes #5188.

[jgm]

Is it worth adding something that checks to make sure that lvl - shift is positive, leaving things alone otherwise?

[tarleb]

Thank you, this is looking good. I love the fact that deleting code seems to add more features.
I think we currently run into problems if someone uses :minlevel 0 (which is fine for org-mode, it demotes first-level headers to top-level paragraphs). Would that be easy to add? If not, simply preventing pandoc from behaving weirdly would be enough for now, I think.

[jgm]

I think the check is needed, but I'll just merge this and add a further commit on top of it.

[leungbk]

Ugh, I'm really sorry for the delay--I've been busier than expected and had some computer problems as well.

Yeah, the == 0 for lvl - shift needs to be there, but I think the < 0 check belongs outside, in the blockFilter function. Right now a negative value would never get passed into shiftHeader. I wasn't familiar with safeRead, and never got around to learning some of the relevant types.

[jgm]

No worries @leungbk! Thanks for the PR. I just didn't want it to languish and be forgotten.

[jgm]

@leungbk @tarleb have a look at commit 710a22e.
This seems to agree with the org exporter when minlevel == 0. (h1 becomes a regular paragraph, h2 becomes h1 -- that's not very useful behavior, but that's how it seems to work.)
It also guarantees we won't get header level < 1.