Remove fetch-depth: 0 by running git fetch instead (#30953)

.github/workflows/test.yml

@@ -36,7 +36,6 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        # The same array lives in test-windows.yml, so make any updates there too.
         test-group:
           [
             content,
@@ -73,9 +72,9 @@ jobs:
           # only for the test groups that we know need the files.
           lfs: ${{ matrix.test-group == 'content' }}
           # Enables cloning the Early Access repo later with the relevant PAT
-          persist-credentials: 'false'
-          # Get the branches so merge_group can make the comparison with git diff
-          fetch-depth: 0
+          # persist-credentials: 'false'
+          # ->> Do we really need this? actions/checkout doesn't use it for the nested example:
+          # https://github.com/actions/checkout#checkout-multiple-repos-nested
 
       - name: Figure out which docs-early-access branch to checkout, if internal repo
         if: ${{ github.repository == 'github/docs-internal' }}
@@ -135,21 +134,31 @@ jobs:
           PR: ${{ github.event.pull_request.number }}
           HEAD: ${{ github.event.pull_request.head.ref || github.event.merge_group.head_ref }}
         run: |
+          # Find the file diff in the pull request or merge group
           # If its a pull request, use the faster call to the GitHub API
           # For push, workflow_dispatch, and merge_group, use git diff
           if [ -n "$PR" ]
           then
+            echo __ running gh pr diff __
             DIFF=`gh pr diff $PR --name-only`
           elif [ -n "$HEAD" ]
           then
+            echo __ running git fetch main __
+            git fetch origin main --depth 1
+            echo __ running git diff __
             DIFF=`git diff --name-only origin/main`
           else
+            echo __ no head, empty diff __
             DIFF=''
           fi
+          # So we can inspect the output
+          echo __ DIFF found __
+          echo $DIFF
 
           # So that becomes a string like `foo.js path/bar.md`
           # Must to do this because the list of files can be HUGE. Especially
           # in a repo-sync when there are lots of translation files involved.
+          echo __ format, write to get_diff_files.txt __
           echo $DIFF | tr '\n' ' ' > get_diff_files.txt
 
       - name: Setup node