Block online change of a vcs

This could work, but the problem is that before changed object is saved
the linked repos still see old vcs leading to horrible mess.  Changing
vcs from the manage.py shell works fine though.
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/models/subproject.py

@@ -1070,6 +1070,15 @@ class SubProject(models.Model, PercentMixin, URLMixin, PathMixin):
             old = SubProject.objects.get(pk=self.id)
             self.check_rename(old)
 
+            if old.vcs != self.vcs:
+                # This could work, but the problem is that before changed
+                # object is saved the linked repos still see old vcs leading
+                # to horrible mess. Changing vcs from the manage.py shell
+                # works fine though.
+                raise ValidationError(
+                    _('Changing version control system is not supported!')
+                )
+
         # Check file format
         if self.file_format not in FILE_FORMATS:
             raise ValidationError(