Doc CI: always use git add --all and git diff --staged

Fixes that some changes were not considered in the sync job.

Author: Bromeon

.github/workflows/doc.yml

@@ -78,11 +78,12 @@ jobs:
 
           find gdnative -name .html -o -type f -print0 | xargs -0 sed -i 's/'"$libVersion"'/master/g'
 
-          if git diff --exit-code --quiet
+          # Add all files to index and look at staged diff
+          git add --all
+          if git diff --exit-code --staged --quiet
           then
             echo "$shortSha introduces no doc changes; skip commit and push."
           else
-            git add --all
             git commit -m "Sync doc from $shortSha
   
             Revision in godot-rust: ${GITHUB_SHA}"