Roll back part of of `--shift-heading-level-by` change.

With positive heading shifts, starting in 2.8 this option caused
metadata titles to be removed and changed to regular headings.
This behavior is incompatible with the old behavior of
`--base-header-level` and breaks old workflows, so with this
commit we are rolling back this change.

Now, there is an asymmetry in positive and negative heading
level shifts:

+ With positive shifts, the metadata title stays the same and
  does not get changed to a heading in the body.
+ With negative shifts, a heading can be converted into the
  metadata title.

I think this is a desirable combination of features, despite
the asymmetry.  One might, e.g., want to have a document
with level-1 section headigs, but render it to HTML with
level-2 headings, retaining the metadata title (which pandoc
will render as a level-1 heading with the default template).

Closes #5957.
Revises #5615.

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