Fix branch handling in formatting workflow

We already checkout the PR's branch, so no need to fetch it again. That also
avoids conflicts with the base branch, if both are called the same.

Also, always refer to the remote's branch directly, then there's no need to
store the branch locally.

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

@@ -15,9 +15,9 @@ 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
+  # GitHub also provides github.event.pull_request.base.sha, but that isn't
+  # always the latest commit on the base branch (e.g. see
+  # https://github.com/AliceO2Group/AliceO2/pull/12499). Using it might lead
   # to false positives in the errors we show.
   BASE_BRANCH: ${{ github.event.pull_request.base.ref }}
   PR_BRANCH: ${{ github.event.pull_request.head.ref }}
@@ -57,7 +57,7 @@ jobs:
         run: |
           git config --global user.email '[email protected]'
           git config --global user.name 'ALICE Action Bot'
-          git fetch origin "$BASE_BRANCH:$BASE_BRANCH" "pull/$PR_NUMBER/head:$PR_BRANCH"
+          git fetch origin "$BASE_BRANCH"
 
       - name: Run clang format
         id: clang_format
@@ -66,7 +66,7 @@ jobs:
           # $BASE_BRANCH is the branch the PR will be merged into, NOT the
           # commit this PR derives from! For that, we need to find the latest
           # common ancestor between the PR and the branch we are merging into.
-          base_commit=$(git merge-base HEAD "$BASE_BRANCH")
+          base_commit=$(git merge-base HEAD "origin/$BASE_BRANCH")
           # Find changed files, ignoring binary files.
           readarray -d '' commit_files < \
             <(git diff -z --diff-filter d --name-only "$base_commit")
@@ -157,10 +157,9 @@ 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 "$BASE_BRANCH:$BASE_BRANCH" "pull/$PR_NUMBER/head:$PR_BRANCH"
+      # Fetch the PR's base branch to find the common ancestor.
+      - name: Update PR branch
+        run: git fetch origin "$BASE_BRANCH"
 
       - name: Check copyright headers
         env:
@@ -183,7 +182,7 @@ 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 "$BASE_BRANCH" \
+            <(git diff -z --diff-filter d --name-only --merge-base "origin/$BASE_BRANCH" \
                        -- '*.cxx' '*.h' '*.C' '*.cmake' '*/CMakeLists.txt')
           # Run copyright notice check. Comment lines start with "//" for C++
           # files and "#" for CMake files.
@@ -311,16 +310,15 @@ 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 "$BASE_BRANCH:$BASE_BRANCH" "pull/$PR_NUMBER/head:$PR_BRANCH"
+      # Fetch the PR's base branch to find the common ancestor.
+      - name: Update PR branch
+        run: git fetch origin "$BASE_BRANCH"
 
       - name: Find bad spacing
         run: |
           # Find changed files, ignoring binary files.
           readarray -d '' files < \
-            <(git diff -z --diff-filter d --name-only --merge-base "$BASE_BRANCH" |
+            <(git diff -z --diff-filter d --name-only --merge-base "origin/$BASE_BRANCH" |
                 while read -rd '' filename; do
                   file -bi "$filename" | grep -q charset=binary ||
                     printf "%s\\0" "$filename"