[Github] Fetch before files changed action on push in docs action

My previous commit 397f1ce switched up
the docs github action to use the GH API for pull requests. However,
this breaks the action when pushing to a branch as the Github API can't
provide diffs for commits. This patch fixes that by creating a local
checkout beforehand on push events for determining the changed files.

Author: boomanaiden154

.github/workflows/docs.yml

@@ -33,6 +33,15 @@ jobs:
     name: "Test documentation build"
     runs-on: ubuntu-latest
     steps:
+      # Don't fetch before checking for file changes to force the file changes
+      # action to use the Github API in pull requests. If it's a push to a
+      # branch we can't use the Github API to get the diff, so we need to have
+      # a local checkout beforehand.
+      - name: Fetch LLVM sources (Push)
+        if: ${{ github.event_name == 'push' }}
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 1
       - name: Get subprojects that have doc changes
         id: docs-changed-subprojects
         uses: tj-actions/changed-files@v39
@@ -50,7 +59,8 @@ jobs:
               - 'libunwind/docs/**'
             libcxx:
               - 'libcxx/docs/**'
-      - name: Fetch LLVM sources
+      - name: Fetch LLVM sources (PR)
+        if: ${{ github.event_name == 'pull_request' }}
         uses: actions/checkout@v4
         with:
           fetch-depth: 1