Add clang-format-diff to github actions

Back again with another attempt at #12446 after 6 years.

Author: sbc100

.clang-format-ignore

@@ -0,0 +1,3 @@
+# ignore third_party code from clang-format checks
+third_party/*
+test/third_party/*

.github/workflows/clang-format-diff.yml

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+name: clang-format-diff
+
+on: [pull_request]
+
+jobs:
+  clang_format_diff:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+      # Fetch all history for all tags and branches
+      with:
+        fetch-depth: 0
+    - name: Install clang-format
+      run: |
+        sudo apt-get install clang-format-19
+        sudo update-alternatives --install /usr/bin/git-clang-format git-clang-format /usr/bin/git-clang-format-19 100
+    - run: tests/clang-format-diff.sh origin/$GITHUB_BASE_REF

tools/maint/clang-format-diff.sh

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+#!/bin/bash
+# Based on binaryen/scripts/clang-format-diff.sh
+# TODO(sbc): Switch to pre-packaged github actions once we find one that
+# is mature enough for our needs.
+
+set -o errexit
+set -o pipefail
+
+if [ -n "$1" ]; then
+  BRANCH="$1"
+elif [ -n "$GITHUB_BASE_REF" ]; then
+  BRANCH="origin/$GITHUB_BASE_REF"
+else
+  BRANCH="@{upstream}"
+fi
+
+MERGE_BASE=$(git merge-base $BRANCH HEAD)
+FORMAT_ARGS=${MERGE_BASE}
+if [ -n "$LLVM_VERSION" ]; then
+  FORMAT_ARGS="--binary=clang-format-${LLVM_VERSION} ${FORMAT_MSG}"
+fi
+
+FORMAT_MSG=$(git clang-format ${FORMAT_ARGS} -q --diff || true)
+if [ -n "$FORMAT_MSG" -a "$FORMAT_MSG" != "no modified files to format" ]
+then
+  echo "Please run git clang-format with clang-format-${LLVM_VERSION} before committing, or apply this diff:"
+  echo
+  # Run git clang-format again, this time without capruting stdout.  This way
+  # clang-format format the message nicely and add color.
+  git clang-format ${FORMAT_ARGS} -q --diff
+  exit 1
+fi