round tripping escaped atx header syntax can result in headers where none were intended

[jjallaire]

Consider this following markdown file headings.md:

- \# First-level heading

- \#\# Second-level heading

- \#\#\# Etc.

If we round trip this from JSON and back to markdown (using pandoc 2.16.2 on MacOS):

pandoc brackets.md --to json | pandoc --from json --to markdown

We get the following output:

-   \# First-level heading

-   ## Second-level heading

-   ##\# Etc.

[jgm]

This is quite strange!

% pandoc -f native -t markdown
[Str "##",Space,Str "hi"]
## hi
% pandoc -f native -t markdown
[Str "###",Space,Str "hi"]
##\# hi

[jgm]

This is due to l. 93 of T.P.Writers.Markdown.Inline, but I have to figure out why that's there!

[jgm]

See commit 5a6399d and #6259.

[jjallaire]

Thanks!