git: use GIT_DISCOVERY_ACROSS_FILESYSTEM for better compatability on NFS servers and shadow repo.

vcsserver/git.py

@@ -636,6 +636,7 b' class GitRemote(object):'
         if 'GIT_DIR' in gitenv:
             del gitenv['GIT_DIR']
         gitenv['GIT_CONFIG_NOGLOBAL'] = '1'
+        gitenv['GIT_DISCOVERY_ACROSS_FILESYSTEM'] = '1'
 
         cmd = [settings.GIT_EXECUTABLE] + _copts + cmd