Unexpected removal of newlines

[rpdelaney]

Describe the bug

context
We use markdown ADRs (mADR) to document engineering decisions. There is a template for an mADR.

We also use mdformat to format markdown files, which is enforced by pre-commit and in CI.

expectation
When running mdformat on the mADR template, I expected that the HTML rendering on github would not dramatically change.

bug
But instead, several lines were joined into a preceding header:

----
+______________________________________________________________________
+
 # These are optional elements. Feel free to remove any of them.
-status: {proposed | rejected | accepted | deprecated | … | superseded by [ADR-0005](0005-example.md)}
-date: {YYYY-MM-DD when the decision was last updated}
-deciders: {list everyone involved in the decision}
-consulted: {list everyone whose opinions are sought (typically subject-matter experts); and with whom there is a two-way communication}
-informed: {list everyone who is kept up-to-date on progress; and with whom there is a one-way communication}
----
+
+## status: {proposed | rejected | accepted | deprecated | … | superseded by [ADR-0005](0005-example.md)} date: {YYYY-MM-DD when the decision was last updated} deciders: {list everyone involved in the decision} consulted: {list everyone whose opinions are sought (typically subject-matter experts); and with whom there is a two-way communication} informed: {list everyone who is kept up-to-date on progress; and with whom there is a one-way communication}

problem
This is a problem for us because doing a recursive mdformat or pre-commit run --all with an mdformat hook will always fail unless we break the template to match mdformat's expected behavior.

Reproduce the bug

$ curl 'https://raw.githubusercontent.com/adr/madr/main/template/adr-template.md' > template.md
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  3103  100  3103    0     0  40925      0 --:--:-- --:--:-- --:--:-- 43704
$ mdformat template.md

List your environment

$ python3 --version
Python 3.10.6
$ mdformat --version
mdformat 0.7.16 (mdformat_tables: 0.4.1, mdformat_gfm: 0.3.5, mdformat_toc: 0.3.0, mdformat_black: 0.1.1)

Relevant pre-commit config:

   - repo: https://github.com/executablebooks/mdformat
     rev: 0.7.16
     hooks:
       - id: mdformat
         additional_dependencies:
           - mdformat-gfm
           - mdformat-toc

[hukkin]

I expected that the HTML rendering on github would not dramatically change.

Quoting https://github.com/executablebooks/mdformat#installing

Note that GitHub's Markdown renderer supports syntax extensions not included in the GFM specification. For full GitHub support do:

   pip install mdformat-gfm mdformat-frontmatter mdformat-footnote

Mdformat is CommonMark compliant by default. You are using frontmatter syntax which isn't included in CommonMark or GFM so you'll need to install mdformat-frontmatter.

[hukkin]

Cross-referencing #332