release-notes.sh: remove line breaks from generated Markdown, for GitHub

GitHub-flavored Markdown translates line breaks to <br/>, which does
not match our intent. Work around that by joining lines when producing
Markdown output.

build_tools/release-notes.sh

@@ -38,6 +38,7 @@ sphinx-build >&2 -j auto \
     -d "$relnotes_tmp/doctree" "$relnotes_tmp/fake-workspace/doc_src" "$relnotes_tmp/out" \
     -D markdown_http_base="https://fishshell.com/docs/$minor_version" \
     -D markdown_uri_doc_suffix=".html" \
+    -D markdown_github_flavored=1 \
     "$@"
 
 # Delete changelog header