Test InvalidGitRepositoryError in repo subdir

A Repo object can of course be constructed from a path to a
directory that is the root of an existing repository, and raises
InvalidGitRepositoryError on a directory that is outside of any
repository. Tests intended to show both conditions already exist.

This adds a test to verify that InvalidGitRepositoryError is also
raised when an attempt is made to construct a Repo object from a
path to a directory that is not the root of a repository but that
is known to be located inside an existing git repository.

test/test_repo.py

@@ -77,9 +77,20 @@ def tearDown(self):
         gc.collect()
 
     def test_new_should_raise_on_invalid_repo_location(self):
+        # Ideally this tests a directory that is outside of any repository. In the rare
+        # case tempfile.gettempdir() is inside a repo, this still passes, but tests the
+        # same scenario as test_new_should_raise_on_invalid_repo_location_within_repo.
         with tempfile.TemporaryDirectory() as tdir:
             self.assertRaises(InvalidGitRepositoryError, Repo, tdir)
 
+    @with_rw_directory
+    def test_new_should_raise_on_invalid_repo_location_within_repo(self, rw_dir):
+        repo_dir = osp.join(rw_dir, "repo")
+        Repo.init(repo_dir)
+        subdir = osp.join(repo_dir, "subdir")
+        os.mkdir(subdir)
+        self.assertRaises(InvalidGitRepositoryError, Repo, subdir)
+
     def test_new_should_raise_on_non_existent_path(self):
         with tempfile.TemporaryDirectory() as tdir:
             nonexistent = osp.join(tdir, "foobar")
@@ -122,7 +133,7 @@ def test_tree_from_revision(self):
         self.assertEqual(tree.type, "tree")
         self.assertEqual(self.rorepo.tree(tree), tree)
 
-        # try from invalid revision that does not exist
+        # Try from an invalid revision that does not exist.
         self.assertRaises(BadName, self.rorepo.tree, "hello world")
 
     def test_pickleable(self):