contest: fetcher: try to work around a git lock issue

After updating git to 2.47 we see random crashes like this:

```
   From https://github.com/linux-netdev/testing
    - [deleted]                   (none)     -> origin/net-next-2025-02-21--00-00
    * [new branch]                net-next-2025-02-26--03-00 -> origin/net-next-2025-02-26--03-00
   error: cannot lock ref 'HEAD': Unable to create '/home/virtme/testing-13/.git/HEAD.lock': File exists.
   Another git process seems to be running in this repository, e.g.
   an editor opened by 'git commit'. Please make sure all processes
   are terminated then try again. If it still fails, a git process
   may have crashed in this repository earlier:
   remove the file manually to continue.
   fatal: unable to update HEAD
   branch 'net-next-2025-02-26--03-00' set up to track 'origin/net-next-2025-02-26--03-00'.
   Traceback (most recent call last):
     File "/opt/nipa/contest/remote/vmksft-p.py", line 376, in <module>
       main()
     File "/opt/nipa/contest/remote/vmksft-p.py", line 371, in main
       f.run()
     File "/opt/nipa/contest/remote/lib/fetcher.py", line 157, in run
       self._run_once()
     File "/opt/nipa/contest/remote/lib/fetcher.py", line 143, in _run_once
       subprocess.run('git checkout ' + to_test["branch"],
     File "/usr/lib64/python3.11/subprocess.py", line 571, in run
       raise CalledProcessError(retcode, process.args,
   subprocess.CalledProcessError: Command 'git checkout net-next-2025-02-26--03-00' returned non-zero exit status 128.
```

Note that the initial sleep seems necessary.

Signed-off-by: Jakub Kicinski <[email protected]>

Author: kuba-moo

contest/remote/lib/fetcher.py

@@ -139,7 +139,17 @@ def _run_once(self):
 
         # For now assume URL is in one of the remotes
         subprocess.run('git fetch --all --prune', cwd=self._tree_path,
-                       shell=True)
+                       shell=True, check=True)
+
+        # After upgrading git 2.40.1 -> 2.47.1 CI hits a race in git,
+        # where tree is locked, even though previous command has finished.
+        # We need to sleep a bit and then wait for the lock to go away.
+        time.sleep(0.2)
+        lock_path = os.path.join(self._tree_path, '.git/HEAD.lock')
+        while os.path.exists(lock_path):
+            print("HEAD is still locked! Sleeping..")
+            time.sleep(0.2)
+
         subprocess.run('git checkout ' + to_test["branch"],
                        cwd=self._tree_path, shell=True, check=True)