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.

MANUAL.txt

@@ -493,14 +493,15 @@ header when requesting a document from a URL:
     headings become level 1 headings, and level 3 headings
     become level 2 headings.  Headings cannot have a level
     less than 1, so a heading that would be shifted below level 1
-    becomes a regular paragraph.  Exception: with a shift of -1,
-    a level-1 heading at the beginning of the document
-    replaces the metadata title. Conversely, with a shift
-    of +1, a nonempty metadata title becomes a level-1 heading at
-    the beginning of the document.  `--shift-heading-level-by=-1`
+    becomes a regular paragraph.  Exception: with a shift of -N,
+    a level-N heading at the beginning of the document
+    replaces the metadata title. `--shift-heading-level-by=-1`
     is a good choice when converting HTML or Markdown documents that
     use an initial level-1 heading for the document title and
-    level-2+ headings for sections.
+    level-2+ headings for sections. `--shift-heading-level-by=1`
+    may be a good choice for converting Markdown documents that
+    use level-1 headings for sections to HTML, since pandoc uses
+    a level-1 heading to render the document title.
 
 `--base-header-level=`*NUMBER*
 

src/Text/Pandoc/Shared.hs

@@ -641,12 +641,6 @@ headerShift n (Pandoc meta (Header m _ ils : bs))
   | n < 0
   , m + n == 0 = headerShift n $
                  B.setTitle (B.fromList ils) $ Pandoc meta bs
-headerShift n (Pandoc meta bs)
-  | n > 0
-  , not (null (docTitle meta))
-    = Pandoc meta' (Header n nullAttr (docTitle meta) : bs')
- where
-   Pandoc meta' bs' = headerShift n $ B.deleteMeta "title" $ Pandoc meta bs
 headerShift n (Pandoc meta bs) = Pandoc meta (walk shift bs)
  where
    shift :: Block -> Block

test/command/shift-heading-level-by.md

@@ -8,9 +8,8 @@ title: My title
 
 ## Second
 ^D
-Pandoc (Meta {unMeta = fromList []})
-[Header 1 ("",[],[]) [Str "My",Space,Str "title"]
-,Header 2 ("first-heading",[],[]) [Str "First",Space,Str "heading"]
+Pandoc (Meta {unMeta = fromList [("title",MetaInlines [Str "My",Space,Str "title"])]})
+[Header 2 ("first-heading",[],[]) [Str "First",Space,Str "heading"]
 ,Header 3 ("second",[],[]) [Str "Second"]]
 ```