[misc] Improve format server stability (taichi-dev#672)

Force delete when user reuses branch name

misc/format_server.py

@@ -81,7 +81,7 @@ def do_GET(self):
         ssh_url = head['repo']['ssh_url']
         self.exec(f'git remote add {user_id} {ssh_url}')
         self.exec(f'git fetch {user_id} {branch_name}')
-        self.exec(f'git branch -d {user_id}-{branch_name}')
+        self.exec(f'git branch -D {user_id}-{branch_name}')
         self.exec(
             f'git checkout -b {user_id}-{branch_name} {user_id}/{branch_name}')
         commits = self.exec(