Make sure we have the base branch

Author: thomasspriggs

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

@@ -26,6 +26,8 @@ jobs:
           echo "Pull request's branch is: ${PR_BRANCH}"
           clang-format-7 --version
           while read file ; do EXCLUDES+="':(top,exclude)$file' " ; done < .clang-format-ignore
+          git checkout ${BASE_BRANCH}
+          git checkout -
           MERGE_BASE=$(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