`Repository.diff(cached=True)` does not list diffs for indexed files

This is with pygit2 version 0.24.0.

$ git init
Initialized empty Git repository in /tmp/minimal/.git/
$ touch foo
$ git add foo
$ git commit -am "First commit"
[master (root-commit) 3551222] First commit
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 foo
$ echo "foobar" >> foo                                                                                                                                                                                                                                                                                 
$ git add foo
$ git diff --cached
diff --git a/foo b/foo
index e69de29..323fae0 100644
--- a/foo
+++ b/foo
@@ -0,0 +1 @@
+foobar
$ python
>>> import pygit2
>>> repo = pygit2.Repository('.')
>>> list(repo.diff(cached=True))
[]

Start by reproducing the documented pygit2.Repository('.') and Repository.diff(cached=True) example, comparing its result with git diff --cached. Trace the cached diff entry point and related tests in pygit2, then verify that indexed-file changes are returned and match the command-line diff.