InvalidSpecError for `repo.checkout("rev")` when `git checkout rev` works fine

To reproduce:

git clone https://src.fedoraproject.org/rpms/python-pymongo.git
cd python-pymongo
python3
>>> import pygit2
>>> pyrepo = pygit2.Repository("./")
>>> pyrepo.checkout("a3432e746082d0c96811cbf87aa7ae8e7fe62bc8")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.11/site-packages/pygit2/repository.py", line 420, in checkout
    reference = self.lookup_reference(refname)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
_pygit2.InvalidSpecError: a3432e746082d0c96811cbf87aa7ae8e7fe62bc8: the given reference name 'a3432e746082d0c96811cbf87aa7ae8e7fe62bc8' is not valid
>>> (ctrl-d)
[adamw@xps13a python-pymongo (rawhide)]$ git checkout a3432e746082d0c96811cbf87aa7ae8e7fe62bc8
Note: switching to 'a3432e746082d0c96811cbf87aa7ae8e7fe62bc8'.
[...]
HEAD is now at a3432e7 Update to 3.4.0 (#1400227).

so, checkout of that commit works fine with git checkout but not pygit2 checkout. I've no idea why not. Not sure if this is a pygit2 or libgit2 issue.

libgit2 / pygit2

InvalidSpecError for `repo.checkout("rev")` when `git checkout rev` works fine #1217