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.
Deprecate --base-heading-level.
The new option does everything the old one does, but also
allows negative shifts. It also promotes the document
metadata (if not null) to a level-1 heading with a +1 shift,
and demotes an initial level-1 heading to document metadata
with a -1 shift. This supports converting documents that
use an initial level-1 heading for the document title.
Closes#5615.
