Simplify code for setting commit message

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/views/edit.py

@@ -304,13 +304,12 @@ def handle_translate(translation, request, user_locked,
         )
     elif not user_locked:
         # Custom commit message
-        if ('commit_message' in request.POST
-                and request.POST['commit_message']
-                != unit.translation.commit_message):
+        message = requst.POST.get('commit_message')
+        if message and message != unit.translation.commit_message):
             # Commit pending changes so that they don't get new message
             unit.translation.commit_pending(request, request.user)
             # Store new commit message
-            unit.translation.commit_message = request.POST['commit_message']
+            unit.translation.commit_message = message
             unit.translation.save()
 
         go_next = perform_translation(unit, form, request)