Add missing post-release script

.github/scripts/merge-change-log-after-release.sh

@@ -0,0 +1,44 @@
+#!/bin/bash -e
+
+# this script merges release notes for $VERSION into CHANGELOG.md
+# the release date for $VERSION should be available in $RELEASE_DATE
+# and the release notes for $VERSION should be available in /tmp/changelog-section.md
+
+if [[ $VERSION =~ ^[0-9]+\.[0-9]+\.0 ]]; then
+  # this was not a patch release, so the version exists already in the CHANGELOG.md
+
+  # update the release date
+  sed -Ei "s/## Version $VERSION .*/## Version $VERSION ($RELEASE_DATE)/" CHANGELOG.md
+
+  # the entries are copied over from the release branch to support workflows
+  # where change log entries may be updated after preparing the release branch
+
+  {
+    # copy the portion above the release, up to and including the heading
+    sed -n "0,/^## Version $VERSION /p" CHANGELOG.md
+    # copy the release notes for $VERSION
+    cat /tmp/changelog-section.md
+    # copy the portion below the release
+    sed -n "0,/^## Version $VERSION /d;0,/^## Version /{/^## Version/!d};p" CHANGELOG.md
+  } > /tmp/CHANGELOG.md
+
+  # update the real CHANGELOG.md
+  cp /tmp/CHANGELOG.md CHANGELOG.md
+
+else
+  # this was a patch release, so the version does not exist already in the CHANGELOG.md
+
+  {
+    # copy the portion above the top-most release, not including the heading
+    sed -n "0,/^## Version /{ /^## Version /!p }" CHANGELOG.md
+    # add the heading
+    echo "## Version $VERSION ($RELEASE_DATE)"
+    # copy the release notes for $VERSION
+    cat /tmp/changelog-section.md
+    # copy the portion starting from the top-most release
+    sed -n "/^## Version /,\$p" CHANGELOG.md
+  } > /tmp/CHANGELOG.md
+
+  # update the real CHANGELOG.md
+  cp /tmp/CHANGELOG.md CHANGELOG.md
+fi