support setting a detatched HEAD

src/repository.c

@@ -179,13 +179,22 @@ Repository_head__get__(Repository *self)
 }
 
 int
-Repository_head__set__(Repository *self, PyObject *py_refname)
+Repository_head__set__(Repository *self, PyObject *py_val)
 {
     int err;
+    if (PyObject_TypeCheck(py_val, &OidType)) {
+        git_oid oid;
+        py_oid_to_git_oid(py_val, &oid);
+        err = git_repository_set_head_detached(self->repo, &oid, NULL, NULL);
+        if (err < 0) {
+            Error_set(err);
+            return -1;
+        }
+    } else {
         const char *refname;
         PyObject *trefname;
 
-    refname = py_str_borrow_c_str(&trefname, py_refname, NULL);
+        refname = py_str_borrow_c_str(&trefname, py_val, NULL);
         if (refname == NULL)
             return -1;
 
@@ -195,6 +204,7 @@ Repository_head__set__(Repository *self, PyObject *py_refname)
             Error_set_str(err, refname);
             return -1;
         }
+    }
 
     return 0;
 }

test/test_repository.py

@@ -70,6 +70,15 @@ class RepositoryTest(utils.BareRepoTestCase):
         self.assertFalse(self.repo.head_is_unborn)
         self.assertFalse(self.repo.head_is_detached)
 
+    def test_set_head(self):
+        # Test setting a detatched HEAD.
+        self.repo.head = Oid(hex=PARENT_SHA)
+        self.assertEqual(self.repo.head.target.hex, PARENT_SHA)
+        # And test setting a normal HEAD.
+        self.repo.head = "refs/heads/master"
+        self.assertEqual(self.repo.head.name, "refs/heads/master")
+        self.assertEqual(self.repo.head.target.hex, HEAD_SHA)
+
     def test_read(self):
         self.assertRaises(TypeError, self.repo.read, 123)
         self.assertRaisesWithArg(KeyError, '1' * 40, self.repo.read, '1' * 40)