housekeeping: make release note generator more flexible (via #5266)

release/get-changelog.sh

-echo '**Interface changes**: ???'
-echo ''
-echo '**Changelog**:'
-./node_modules/.bin/conventional-changelog -p angular | tail -n +3
+echo "_No release summary included._\n\n#### Changelog\n"
+
+PREV_RELEASE_REF=$(git log --pretty=oneline | grep ' release: ' | head -n 2 | tail -n 1 | cut -f 1 -d " ")
+
+git log --pretty=oneline $PREV_RELEASE_REF..HEAD | awk '{ $1=""; print}' | sed -e 's/^[ \t]*//' | sed 's/^feat/0,feat/' | sed 's/^improve/1,improve/' | sed 's/^fix/2,fix/'| sort | sed 's/^[0-2],//' | sed 's/^/* /'