ValueError: Problem accessing HEAD branch

Hi! After updating my images today, nbgitpuller started to throw this error when starting from an url which worked previously.

Here is the url (generated with the generator) and the error message:

https://mybinder.org/v2/gh/OGGM/binder/stable?urlpath=git-pull%3Frepo%3Dhttps%253A%252F%252Fgithub.com%252FOGGM%252Ftutorials%26urlpath%3Dlab%252Ftree%252Ftutorials%252F%26branch%3Dstable

```
Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.9/site-packages/nbgitpuller/pull.py", line 120, in resolve_default_branch
head_branch = subprocess.run(
File "/srv/conda/envs/notebook/lib/python3.9/subprocess.py", line 528, in run
raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['git', 'ls-remote', '--symref', '--', 'https://github.com/OGGM/tutorials', 'HEAD']' returned non-zero exit status 128.
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.9/site-packages/nbgitpuller/handlers.py", line 76, in get
gp = GitPuller(repo, repo_dir, branch=branch, depth=depth, parent=self.settings['nbapp'])
File "/srv/conda/envs/notebook/lib/python3.9/site-packages/nbgitpuller/pull.py", line 76, in __init__
self.branch_name = self.resolve_default_branch()
File "/srv/conda/envs/notebook/lib/python3.9/site-packages/nbgitpuller/pull.py", line 136, in resolve_default_branch
raise ValueError(m)
ValueError: Problem accessing HEAD branch: https://github.com/OGGM/tutorials
```