Markdown writer: avoid HTML fallbacks in the generated TOC.

The generated table of contents usually has IDs for each TOC link,
allowing to link back to specific parts of the TOC. However, this leads
to unidiomatic markup in formats like gfm, which do not support
attributes on links and hence fall back to HTML. The IDs on TOC items
are now removed in that case, leading to more aesthetic TOCs.

Closes: #8131

1 files changed, 41 insertions, 0 deletions

diff --git a/test/command/8131.md b/test/command/8131.md
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+# TOC in gfm contains no HTML by default
+```
+% pandoc --to=gfm --toc --standalone
+# Head
+
+Content
+^D
+- [Head](#head)
+
+# Head
+
+Content
+```
+
+# Same in Markdown if link_attributes extension is disabled
+```
+% pandoc --to=markdown-link_attributes --toc --standalone
+# Head
+
+Content
+^D
+-   [Head](#head)
+
+# Head
+
+Content
+```
+
+# IDs are added to TOC with the "attributes" CommonMark extension
+```
+% pandoc --to=commonmark+gfm_auto_identifiers+attributes --toc -s
+# Nam a sapien
+
+Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
+^D
+- [Nam a sapien](#nam-a-sapien){#toc-nam-a-sapien}
+
+# Nam a sapien
+
+Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
+```