add a test for "Do not crash when cloning HEAD-less repo" with empty.git test data

Conflicts:
	dulwich/tests/test_repository.py

Signed-Off-By: Jelmer Vernooij <jelmer@samba.org>

dulwich/tests/data/repos/empty.git/HEAD

@@ -0,0 +1 @@
+ref: refs/heads/master

dulwich/tests/data/repos/empty.git/config

@@ -0,0 +1,7 @@
+[core]
+	repositoryformatversion = 0
+	filemode = false
+	bare = true
+	symlinks = false
+	ignorecase = true
+	hideDotFiles = dotGitOnly

dulwich/tests/data/repos/empty.git/objects/info/.gitignore

@@ -0,0 +1,2 @@
+*
+!.gitignore

dulwich/tests/data/repos/empty.git/objects/pack/.gitignore

@@ -0,0 +1,2 @@
+*
+!.gitignore

dulwich/tests/data/repos/empty.git/refs/heads/.gitignore

@@ -0,0 +1,2 @@
+*
+!.gitignore

dulwich/tests/data/repos/empty.git/refs/tags/.gitignore

@@ -0,0 +1,2 @@
+*
+!.gitignore

dulwich/tests/test_repository.py

@@ -303,6 +303,19 @@ class RepositoryTests(TestCase):
                 'b0931cadc54336e78a1d980420e3268903b57a50',
             }, t.refs.as_dict())
 
+    def test_clone_empty(self):
+        """Test clone() doesn't crash if HEAD points to a non-existing ref.
+
+        This simulates cloning server-side bare repository either when it is
+        still empty or if user renames master branch and pushes private repo
+        to the server.
+        Non-bare repo HEAD always points to an existing ref.
+        """
+        r = self._repo = open_repo('empty.git')
+        tmp_dir = tempfile.mkdtemp()
+        self.addCleanup(shutil.rmtree, tmp_dir)
+        r.clone(tmp_dir, mkdir=False, bare=True)
+
     def test_merge_history(self):
         r = self._repo = open_repo('simple_merge.git')
         shas = [e.commit.id for e in r.get_walker()]