Add github actions clang-format job

Author: thomasspriggs

.github/workflows/pull-request-checks.yaml

@@ -4,6 +4,42 @@ on:
     branches: [ develop ]
 
 jobs:
+  check-clang-format:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          submodules: true
+      - name: Fetch dependencies
+        env:
+          # This is needed in addition to -yq to prevent apt-get from asking for
+          # user input
+          DEBIAN_FRONTEND: noninteractive
+        run: |
+          sudo apt-get install -yq clang-format-7
+      - name: Check updated lines of code match clang-format-7 style
+        env:
+          BASE_BRANCH: ${{ github.base_ref }}
+          PR_BRANCH: ${{ github.ref }}
+        run: |
+          echo "Pull request's base branch is: ${BASE_BRANCH}"
+          echo "Pull request's branch is: ${PR_BRANCH}"
+          clang-format-7 --version
+          while read file ; do EXCLUDES+="':(top,exclude)$file' " ; done < .clang-format-ignore
+          MERGE_BASE=$(git merge-base ${BASE_BRANCH} ${PR_BRANCH})
+          echo "Checking for formatting errors introduced since $MERGE_BASE"
+          # use eval so that quotes (as inserted into $EXCLUDES above) are not interpreted as parts of file names
+          eval git-clang-format-7 --binary clang-format-7 $MERGE_BASE -- $EXCLUDES
+          git diff > formatted.diff
+          if [[ -s formatted.diff ]] ; then
+            echo 'Formatting error! The following diff shows the required changes'
+            echo 'Use the raw log to get a version of the diff that preserves spacing'
+            cat formatted.diff
+            exit 1
+          fi
+          echo 'No formatting errors found'
+          exit 0
+
   check-macos-10_15-cmake-clang:
     runs-on: macos-10.15
     steps: