checking out a detached HEAD breaks Repo.head.reference

[tiltingpenguin]

While building this package for openSUSE and running the testsuite we encountered a lot of test failures like this

self = <git.HEAD "HEAD">

    def _get_reference(self) -> "Reference":
        """
        :return:
            :class:`~git.refs.reference.Reference` object we point to

        :raise TypeError:
            If this symbolic reference is detached, hence it doesn't point to a
            reference, but to a commit.
        """
        sha, target_ref_path = self._get_ref_info(self.repo, self.path)
        if target_ref_path is None:
>           raise TypeError("%s is a detached symbolic reference as it points to %r" % (self, sha))
E           TypeError: HEAD is a detached symbolic reference as it points to '3481da9618e69063464c94447167d83bd45505a9'

git/refs/symbolic.py:446: TypeError

It seems that this only occurs when checking out a revision instead of a branch, aka when the ref in .git/HEAD points to a commit instead of something like ref: refs/heads/master.

Repo.head.reference in git/repo/base.py fails to resolve references to commit hashes which I would expect to work.

Also there is an unrelated problem where TestRemote.test_base hardcodes the branch name as "master" which leads to the test failing when checking out any other branch.

[Vogtinator]

To reproduce, run git checkout $(git describe HEAD). Then pytest fails with failures like

AILED test/test_base.py::TestBase::test_object_resolution - TypeError: HEAD is a detached symbolic reference as it points to 'b62e91be897f687a622e40cd1e59c33dce45f124'
FAILED test/test_docs.py::Tutorials::test_init_repo_object - TypeError: HEAD is a detached symbolic reference as it points to 'b62e91be897f687a622e40cd1e59c33dce45f124'
FAILED test/test_refs.py::TestRefs::test_is_valid - TypeError: HEAD is a detached symbolic reference as it points to 'b62e91be897f687a622e40cd1e59c33dce45f124'
FAILED test/test_refs.py::TestRefs::test_reflog - TypeError: HEAD is a detached symbolic reference as it points to 'b62e91be897f687a622e40cd1e59c33dce45f124'
FAILED test/test_repo.py::TestRepo::test_head - TypeError: HEAD is a detached symbolic reference as it points to 'b62e91be897f687a622e40cd1e59c33dce45f124'
FAILED test/test_repo.py::TestRepo::test_heads_should_populate_head_data - TypeError: HEAD is a detached symbolic reference as it points to 'b62e91be897f687a622e40cd1e59c33dce45f124'
FAILED test/test_repo.py::TestRepo::test_rev_parse - AssertionError: <git.Commit "b62e91be897f687a622e40cd1e59c33dce45f124"> == <git.Commit "b62e91be897f687a622e40cd1e59c33dce45f124">