simplevcs: fix a typo when running detection of vcs backend.

- the flag should be explicit_scm which skips detection of VCS systems.
This improves the speed of calls by not trying to look for a valid vcs.

rhodecode/lib/middleware/simplevcs.py

@@ -222,7 +222,7 b' class SimpleVCS(object):'
                 repo_name, db_repo.repo_type, scm_type)
             return False
 
-        return is_valid_repo(repo_name, base_path, expect_scm=scm_type)
+        return is_valid_repo(repo_name, base_path, explicit_scm=scm_type)
 
     def valid_and_active_user(self, user):
         """