Add workflow to check for CHANGELOG update

.github/workflows/changelog-check.yaml

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+name: Check CHANGELOG.md updated
+on:
+  pull_request:
+    types: [opened, synchronize, reopened, edited]
+    branches: [main]
+jobs:
+  # Check that the CHANGELOG is updated by the pull request. This can be
+  # disabled by adding a trailer line of the following form to the
+  # pull request message:
+  #
+  #    Disable-Check: force-changelog-changed
+  #
+  # The check is case-insensitive and ignores other contents on the
+  # line as well, so it is possible to add several different checks if
+  # that is necessary.
+  #
+  # It is assumed that the trailer is following RFC2822 conventions,
+  # but this is currently not enforced.
+  check_changelog_update:
+    name: Check for CHANGELOG.md changes
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout source
+        uses: actions/checkout@v2
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 0
+      - name: Check CHANGELOG.md updated by pull request
+        shell: bash --norc --noprofile {0}
+        run: |
+          diff=$(git diff --name-status origin/main | grep "CHANGELOG.md")
+          cat << "EOF" | egrep -qsi '^disable-check:.*\<force-changelog-changed\>'
+          ${{ github.event.pull_request.body }}
+          EOF
+          if [[ $? -ne 0 ]]; then
+            if [[ $diff == '' ]]; then
+              echo
+              echo "CHANGELOG.md not updated by pull request"
+              echo
+              echo "To disable changelog updated check, add this trailer to pull request message:"
+              echo
+              echo "Disable-check: force-changelog-changed"
+              echo
+              echo "Trailers follow RFC2822 conventions, so no whitespace"
+              echo "before field name and the check is case-insensitive for"
+              echo "both the field name and the field body."
+              exit 1
+            fi
+          fi

build-13/.gitattributes

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+# to avoid frequent conflicts
+CHANGELOG.md text merge=union