Always use branches (not SHAs) for formatting workflow

The SHAs that GitHub provides might be outdated, so we'd see lots more changed
files than are actually in the given PR.

By using the branch name (and fetching branches after cloning, for good
measure), we should always get the latest commits in question.

.github/workflows/c++-code-formatting.yml

@@ -15,11 +15,13 @@ name: C++ code formatting reusable workflow
 permissions: {}
 
 env:
+  # GitHub also provides github.event.pull_request.{head,base}.sha, but these
+  # aren't always the latest commits on their respective branches (e.g. see
+  # https://github.com/AliceO2Group/AliceO2/pull/12499). Using them might lead
+  # to false positives in the errors we show.
   BASE_BRANCH: ${{ github.event.pull_request.base.ref }}
   PR_BRANCH: ${{ github.event.pull_request.head.ref }}
   PR_NUMBER: ${{ github.event.pull_request.number }}
-  HEAD_SHA: ${{ github.event.pull_request.head.sha }}
-  BASE_SHA: ${{ github.event.pull_request.base.sha }}
 
 jobs:
   clang-format:
@@ -61,14 +63,13 @@ jobs:
         id: clang_format
         run: |
           set -x
-          # $BASE_SHA is the latest commit on the branch the PR will be merged
-          # into, NOT the commit this PR derives from! For that, we need to find
-          # the latest common ancestor between the PR and the branch we are
-          # merging into.
-          base_commit=$(git merge-base HEAD "$BASE_SHA")
+          # $BASE_BRANCH is the branch the PR will be merged into, NOT the
+          # commit this PR derives from! For that, we need to find the latest
+          # common ancestor between the PR and the branch we are merging into.
+          base_commit=$(git merge-base HEAD "$BASE_BRANCH")
           # Find changed files, ignoring binary files.
           readarray -d '' commit_files < \
-            <(git diff -z --diff-filter d --name-only --merge-base "$BASE_SHA")
+            <(git diff -z --diff-filter d --name-only "$base_commit")
           [ ${#commit_files[@]} -gt 0 ] || { echo "No files to check"; exit 0; }
 
           # Check for invalid file extensions for C++ code.
@@ -156,6 +157,11 @@ jobs:
           # many commits back that point is.
           fetch-depth: 0
 
+      # Fetch the PR's head commit to find the common ancestor.
+      - name: Fetch PR branch
+        run: |
+          git fetch origin "$BASE_BRANCH" "pull/$PR_NUMBER/head:$PR_BRANCH"
+
       - name: Check copyright headers
         env:
           # The expected copyright notice. Comment markers ("//" or "#") are
@@ -177,7 +183,7 @@ jobs:
           # Find changed C++ and CMake files. Keep the file extensions in sync
           # with the ones in the "case" statement below!
           readarray -d '' files < \
-            <(git diff -z --diff-filter d --name-only --merge-base "$BASE_SHA" \
+            <(git diff -z --diff-filter d --name-only --merge-base "$BASE_BRANCH" \
                        -- '*.cxx' '*.h' '*.C' '*.cmake' '*/CMakeLists.txt')
           # Run copyright notice check. Comment lines start with "//" for C++
           # files and "#" for CMake files.
@@ -314,7 +320,7 @@ jobs:
         run: |
           # Find changed files, ignoring binary files.
           readarray -d '' files < \
-            <(git diff -z --diff-filter d --name-only --merge-base "$BASE_SHA" |
+            <(git diff -z --diff-filter d --name-only --merge-base "$BASE_BRANCH" |
                 while read -rd '' filename; do
                   file -bi "$filename" | grep -q charset=binary ||
                     printf "%s\\0" "$filename"