release workflow: resolve relative references in changelog properly

Instead of having sphinx-build only build CHANGELOG.rst, build the
entire thing, so relative references (":doc:", ":ref:") can be resolved
properly.  Replace our sed-based hacks with 'markdown_http_base'.

build_tools/release-notes.sh

@@ -5,27 +5,37 @@ set -e
 workspace_root=$(dirname "$0")/..
 
 relnotes_tmp=$(mktemp -d)
-mkdir "$relnotes_tmp/src" "$relnotes_tmp/out"
+mkdir -p "$relnotes_tmp/fake-workspace" "$relnotes_tmp/out"
+(
+    cd "$workspace_root"
+    cp -r doc_src CONTRIBUTING.rst README.rst "$relnotes_tmp/fake-workspace"
+)
 version=$(sed 's,^fish \(\S*\) .*,\1,; 1q' "$workspace_root/CHANGELOG.rst")
 minor_version=${version%.*}
 changelog_for_this_version=$(awk <"$workspace_root/CHANGELOG.rst" '
     /^===/ { if (v++) { exit } }
     { print }
-' | sed '$d')
-# Also fix up any relative references to other documentation files.
-# Also remove spurious transitions at the end of the document.
+' | sed '$d'
+)
+# Remove spurious transitions at the end of the document.
 printf %s "$changelog_for_this_version" |
-    sed >"$relnotes_tmp/src"/index.rst \
-        -e 's,:doc:`\(.*\) <\([^>]*\)>`,`\1 <https://fishshell.com/docs/'"$minor_version"'/\2.html>`_,g' \
-        -e 's,:envvar:`\([^`]*\)`,``$\1``,g' \
-        -e '$s/^----*$//'
+    sed -e '$s/^----*$//' >"$relnotes_tmp/fake-workspace"/CHANGELOG.rst
 
 # Use "-j 1" because sphinx-markdown-builder is not marked concurrency-safe.
 sphinx-build >&2 -j 1 \
     -W -E -b markdown -c "$workspace_root/doc_src" \
-    -d "$relnotes_tmp/doctree" "$relnotes_tmp/src" $relnotes_tmp/out "$@"
+    -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" \
+    "$@"
+
+# Delete changelog header
+sed -n 1p "$relnotes_tmp/out/relnotes.md" | grep -Fxq "# Release notes"
+sed -n 2p "$relnotes_tmp/out/relnotes.md" | grep -Fxq ''
+sed -i 1,2d "$relnotes_tmp/out/relnotes.md"
+
 {
-    cat "$relnotes_tmp/out/index.md" - <<EOF
+    cat "$relnotes_tmp/out/relnotes.md" - <<EOF
 
 --