[storages] Try to download suffixed keys in precommit events

Since the unsuffixed key is saved in database, a precommit event
fetching an edited attribute doesn't download the new value.

cubicweb_s3storage/storages.py

@@ -51,6 +51,14 @@ class S3Storage(Storage):
         """see docstring for prototype, which vary according to is_source_callback
         """
         key = source.binary_to_str(value).decode('utf-8')
+        if cnx.commit_state == 'precommit':
+            # download suffixed key if it exists
+            # FIXME need a way to check that the attribute is actually edited
+            try:
+                suffixed_key = self.suffixed_key(key)
+                return self.download(suffixed_key)
+            except Exception:
+                pass
         try:
             return self.download(key)
         except Exception as ex: