Solution 1:

The best way to do this is by running the command:

git diff --name-only --cached

When you check the manual you will likely find the following:

--name-only
    Show only names of changed files.

And on the example part of the manual:

git diff --cached
    Changes between the index and your current HEAD.

Combined together, you get the changes between the index and your current HEAD and Show only names of changed files.

--staged is also available as an alias for --cached above in more recent Git versions.

Solution 2:

You can try using git ls-files -s