Reland "[Github] Fetch all commits in PR for code formatting checks (llvm#69766)"

This commit relands 4aa12af. This was
originally reverted as it caused fetch failures due to the usage of the
wrong ref. I didn't catch this in testing as it was attempting to check
the branch out from origin, where it didn't exist, but did on my fork as
I never tested any cross-repo PRs.

Author: boomanaiden154

.github/workflows/pr-code-format.yml

@@ -7,17 +7,37 @@ jobs:
   code_formatter:
     runs-on: ubuntu-latest
     steps:
-      - name: Fetch LLVM sources
-        uses: actions/checkout@v4
-        with:
-          fetch-depth: 2 # Fetches only the last 2 commits
-
+      # Get changed files before checking out the repository to force the action
+      # to analyze the diff from the Github API rather than looking at the
+      # shallow clone and erroring out, which is significantly more prone to
+      # failure.
       - name: Get changed files
         id: changed-files
         uses: tj-actions/changed-files@v39
         with:
           separator: ","
-          fetch_depth: 2000 # Fetches only the last 2000 commits
+
+      - name: Calculate number of commits to fetch
+        run: echo "PR_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+
+      - name: Fetch PR sources
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: ${{ env.PR_FETCH_DEPTH }}
+          path: pr-sources
+
+      # We need to make sure that we aren't executing/using any code from the
+      # PR for security reasons as we're using pull_request_target. Checkout
+      # the target branch with the necessary files.
+      - name: Fetch LLVM Sources
+        uses: actions/checkout@v4
+        with:
+          sparse-checkout: |
+            llvm/utils/git/requirements_formatting.txt
+            llvm/utils/git/code-format-helper.py
+          sparse-checkout-cone-mode: false
+          path: llvm-sources
 
       - name: "Listed files"
         run: |
@@ -34,21 +54,21 @@ jobs:
         with:
           python-version: '3.11'
           cache: 'pip'
-          cache-dependency-path: 'llvm/utils/git/requirements_formatting.txt'
+          cache-dependency-path: 'llvm-sources/llvm/utils/git/requirements_formatting.txt'
 
       - name: Install python dependencies
-        run: pip install -r llvm/utils/git/requirements_formatting.txt
+        run: pip install -r llvm-sources/llvm/utils/git/requirements_formatting.txt
 
       - name: Run code formatter
         env:
           GITHUB_PR_NUMBER: ${{ github.event.pull_request.number }}
-          START_REV: ${{ github.event.pull_request.base.sha }}
-          END_REV: ${{ github.event.pull_request.head.sha }}
+          PR_DEPTH: ${{ github.event.pull_request.commits }}
           CHANGED_FILES: ${{ steps.changed-files.outputs.all_changed_files }}
+        working-directory: ./pr-sources
         run: |
-          python llvm/utils/git/code-format-helper.py \
+          python ../llvm-sources/llvm/utils/git/code-format-helper.py \
             --token ${{ secrets.GITHUB_TOKEN }} \
             --issue-number $GITHUB_PR_NUMBER \
-            --start-rev $START_REV \
-            --end-rev $END_REV \
+            --start-rev HEAD~$PR_DEPTH \
+            --end-rev HEAD \
             --changed-files "$CHANGED_FILES"