use diff-index to prevent commit instead of bash if

preferred because the if suppresses all git commit errors, instead of the one error we need to suppress (commit with no changes)

.github/workflows/snapshot-on-push-master.yml

@@ -115,9 +115,8 @@ jobs:
           git config push.default simple
           git add -A
           # prevent failure when there is nothing to commit
-          if git commit -m "Updated from https://github.com/cc65/cc65/commit/${GITHUB_SHA}" ; then
-            git push
-          fi
+          git diff-index --quiet HEAD || git commit -m "Updated from https://github.com/cc65/cc65/commit/${GITHUB_SHA}"
+          git push
 
     # enter secrets under "repository secrets"
       - name: Upload snapshot to sourceforge