Use linked repository lock for linked projects

We are locking the underlaying repository and not the resource, so we
need shared lock for linked resources.

Fixes #560
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/models/subproject.py

@@ -331,6 +331,9 @@ class SubProject(models.Model, PercentMixin, URLMixin, PathMixin):
         '''
         Returns lock object for current translation instance.
         '''
+        if self.is_repo_link:
+            return self.linked_subproject.repository_lock
+
         if self._repository_lock is None:
             lock_path = os.path.join(
                 self.project.get_path(),