Base auto translation on source string not checksum

It doesn't matter that it's slightly fuzzy (ignoring context) as we want
to make it work for monolingual translations as well (where context/id
will most likely differ).
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/views/edit.py

@@ -641,7 +641,7 @@ def auto_translation(request, project, subproject, lang):
         sources = sources.filter(translation__subproject=subprj)
 
     for unit in units.iterator():
-        update = sources.filter(checksum=unit.checksum)
+        update = sources.filter(source=unit.source)
         if update.exists():
             # Get first entry
             update = update[0]