Last synced revision not always set accuratelly for the translation repositories

[mathjazz]

The code that stores the last synced revision doesn't work as expected when the following conditions are met:

• push to repository fails for some of the locales
• project has multiple repositories
• project doesn't use multi-locale translation repositories

In such case, this code shouldn't update the last synced revision for the repository: https://github.com/mozilla/pontoon/blob/main/pontoon/sync/tasks.py#L387-L391