Fix rebaseline_tests.py script WRT to git submodules. NFC (#23570)

See #23569

Author: sbc100

.github/workflows/ci.yml

@@ -33,6 +33,7 @@ jobs:
       - name: Checkout repo
         uses: actions/checkout@v4
         with:
+          submodules: true
           fetch-depth: 0 # We want access to other branches, specifically `main`
       - name: pip install
         run: |

tools/maint/rebaseline_tests.py

@@ -112,7 +112,8 @@ def main():
 '''
 
   for file in filenames:
-    message += process_changed_file(file)
+    if os.path.isfile(file):
+      message += process_changed_file(file)
 
   message += f'\nAverage change: {statistics.mean(all_deltas):+.2f}% ({min(all_deltas):+.2f}% - {max(all_deltas):+.2f}%)\n'